Grimm was born in Hanau, in Hesse-Kassel (or Hesse-Cassel). His father, who was a lawyer, died while he was a child, and his mother was left with very small means; but her sister, who was lady of the chamber to the Landgravine of Hesse, helped to support and educate her numerous family. Jakob, with his younger brother Wilhelm (born on 24 February 1786), was sent in 1798 to the public school at Kassel.

In 1802 he proceeded to the University of Marburg, where he studied law, a profession for which he had been destined by his father. His brother joined him at Marburg a year later, having just recovered from a long and severe illness, and likewise began the study of law.

Up to this time Jakob Grimm had been actuated only by a general thirst for knowledge and his energies had not found any aim beyond the practical one of making himself a position in life. The first definite impulse came from the lectures of Friedrich Karl von Savigny, the celebrated investigator of Roman law, who, as Wilhelm Grimm himself says in the preface to the Deutsche Grammatik (German Grammar), first taught him to realize what it meant to study any science. Savigny's lectures also awakened in him a love for historical and antiquarian investigation, which forms the basis of all his work. The two men became personally acquainted, and it was in Savigny's well stocked library that Grimm first turned over the leaves of Bodmer's edition of the Old German minnesingers and other early texts, and felt an eager desire to penetrate further into the obscurities and half-revealed mysteries of their language.

In the beginning of 1805 he received an invitation from Savigny, who had moved to Paris, to help him in his literary work. Grimm passed a very happy time in Paris, strengthening his taste for the literatures of the Middle Ages by his studies in the Paris libraries. Towards the close of the year he returned to Kassel, where his mother and Wilhelm had settled, the latter having finished his studies. The next year he obtained a position in the war office with the very small salary of 100 thalers. One of his grievances was that he had to exchange his stylish Paris suit for a stiff uniform and pigtail. But he had full leisure for the prosecution of his studies.

In 1808, soon after the death of his mother, he was appointed superintendent of the private library of Jérôme Bonaparte, King of Westphalia, into which Hesse-Kassel had been incorporated by Napoleon. Jerome appointed him an auditor to the state council, while he retained his other post. His salary was increased in a short interval from 2000 to 4000 francs, and his official duties were hardly more than nominal. After the expulsion of Jerome and the reinstalment of an elector, Grimm was appointed in 1813 secretary of legation, to accompany the Hessian minister to the headquarters of the allied army. In 1814 he was sent to Paris to demand restitution of the books carried off by the French, and in 1814 – 1815 he attended the Congress of Vienna as secretary of legation. On his return he was again sent to Paris on the same errand as before.

Meanwhile Wilhelm had received an appointment in the Kassel library, and in 1816 Jakob was made second librarian under Volkel. Upon the death of Volkel in 1828, the brothers expected to be advanced to the first and second librarianships respectively, and were dissatisfied when the first place was given to Rommel, the keeper of the archives. Consequently, they moved next year to Göttingen where Jakob received the appointment of professor and librarian, and Wilhelm that of under-librarian. Jakob Grimm lectured on legal antiquities, historical grammar, literary history, and diplomatics, explained Old German poems, and commented on the Germania of Tacitus.

At this period he is described as small and lively in figure, with a harsh voice, speaking a broad Hessian dialect. His powerful memory enabled him to dispense with the manuscript on which most German professors relied, and he spoke extemporaneously, referring only occasionally to a few names and dates written on a slip of paper. He regretted that he had begun the work of teaching so late in life, but as a lecturer he was not successful: he had no aptitude for digesting facts and suiting them to the level of comprehension of his students. Even the brilliant, terse, and eloquent passages in his writings lost much of their effect when jerked out in the midst of a long array of dry facts.

In 1837, having been one of the seven professors who signed a protest against the King of Hanover's abrogation of the constitution established some years before, he was dismissed from his professorship and banished from the kingdom of Hanover. He returned to Kassel together with his brother, who had also signed the protest, and remained there until 1840, when they accepted an invitation from the King of Prussia to move to Berlin, where they both received professorships, and were elected members of the Academy of Sciences. Not being under any obligation to lecture, Jakob seldom did so, but together with his brother worked at their great dictionary. During their time in Kassel Jakob regularly attended the meetings of the academy, where he read papers on the most varied subjects. The best known of these are those on Lachmann, Schiller, and his brother Wilhelm (who died in 1859), on old age, and on the origin of language. He also described his impressions of Italian and Scandinavian travel, interspersing his more general observations with linguistic details, as is the case in all his works.

Grimm died in Berlin at the age of 78, working even at the end.

He was never seriously ill, and worked all day without haste and without pause. He was not at all impatient of interruption, but seemed rather to be refreshed by it, returning to his work without effort. He wrote for the press with great rapidity, and hardly ever made corrections. He never revised what he had written, remarking with a certain wonder on his brother, Wilhelm, who read his own manuscripts over again before sending them to press. His temperament was uniformly cheerful, and he was easily amused. Outside his own special work he had a marked taste for botany. The spirit that animated his work is best described by himself at the end of his autobiography:

"Nearly all my labors have been devoted, either directly or indirectly, to the investigation of our earlier language, poetry and laws. These studies may have appeared to many, and may still appear, useless; to me they have always seemed a noble and earnest task, definitely and inseparably connected with our common fatherland, and calculated to foster the love of it. My principle has always been in these investigations to under-value nothing, but to utilize the small for the illustration of the great, the popular tradition for the elucidation of the written monuments."

The purely scientific side of Grimm's character developed slowly. He seems to have felt the want of definite principles of etymology without being able to discover them, and indeed even in the first edition of his grammar (1819) he seemed to be often groping in the dark. As early as 1815 we find August Wilhelm von Schlegel reviewing the Altdeutsche Wälder (a periodical published by the two brothers) very severely, condemning the lawless etymological combinations it contained, and insisting on the necessity of strict philological method and a fundamental investigation of the laws of language, especially in the correspondence of sounds. This criticism is said to have had a considerable influence on the direction of Grimm's studies.

Grimm's scientific character is notable for its combination of breadth and unity. He was as far removed from the narrowness of the specialist who has no ideas or sympathies beyond just one author or corner of science as he was from the shallow dabbler who feverishly attempts to master the details of a half-dozen unrelated pursuits. The same concentration exists within his own special studies. The very foundations of his nature were harmonious; his patriotism and love of historical investigation received their fullest satisfaction in the study of the language, traditions, mythology, laws and literature of his own countrymen and their kin. But from this centre, he pursued his investigations in every direction as far as his instinct allowed. He was equally fortunate in the harmony that existed between his intellectual and moral nature. He cheerfully made the heavy sacrifices that science demands from its disciples, without envy or bitterness; although he lived apart from his fellow men, he was full of human sympathies, and has had a profound influence on the destiny of mankind.

Of all his more general works the boldest and most far-reaching was his Geschichte der deutschen Sprache (History of the German Language), in which the linguistic elements are emphasized. The subject of the work is the history hidden in the words of the German language (the oldest natural history of the Teutonic tribes determined by means of language). For this purpose he laboriously collected the scattered words and allusions found in classical writers, and endeavoured to determine the relationship between the German language and those of the Getae, Thracians, Scythians, and many other nations whose languages were at the time known only through doubtfully identified, often extremely corrupted remains preserved by Greek and Latin authors. Grimm's results have been greatly modified by the wider range of comparison and improved methods of investigation that now characterize linguistics, and many questions he raised will probably remain obscure, but his book's influence has been profound.

Grimm's famous Deutsche Grammatik (German Grammar) was the outcome of his purely philological work. The labors of past generations from the humanists onwards resulted in an enormous collection of materials in the form of text editions, dictionaries, and grammars, although most of it was uncritical and untrustworthy. Something had even been done in the way of the comparison and determination of general laws, and the concept of a comparative Germanic grammar had been clearly grasped by the illustrious Englishman George Hickesby the beginning of the 18th century in his Thesaurus. Ten Kate in Holland had afterwards made valuable contributions to the history and comparison of the Germanic languages. Even Grimm himself did not at first intend to include all the languages in his Grammar, but he soon found that Old High German postulated Gothic, and that the later stages of German could not be understood without the help of other West Germanic varieties including English, and that the rich literature of Scandinavia could not be ignored either. The first edition of the first part of the Grammar (which appeared in 1819), and is now extremely rare, treated of the inflections of all these languages, and included a general introduction, in which he vindicated the importance of an historical study of the German language against the a priori, quasi-philosophical methods then in vogue.

In 1822 this volume appeared in a second edition (really a new work, for, as Grimm himself says in the preface, it cost him little reflection to mow down the first crop to the ground). The wide distance between the two stages of Grimm's development in these two editions is significantly shown by the fact that while the first edition gives only the inflections, in the second volume phonology takes up no fewer than 600 pages, more than half of the whole volume. Grimm had, at last, awakened to the full conviction that all sound philology must be based on rigorous adhesion to the laws of sound change, and he never afterwards swerved from this principle, which gave to all his investigations, even in their boldest flights, that iron-bound consistency, and that force of conviction that distinguishes science from dilettanteism. Prior to Grimm's time, philology was nothing but a more or less laborious and conscientious dilettanteism, with occasional flashes of scientific inspiration.

His advances must be attributed mainly to the influence of his contemporary Rasmus Christian Rask. Rask was born two years later than Grimm, but his remarkable precocity gave him something of a head start. In Grimm's first editions, his Icelandic paradigms are based entirely on Rask's grammar, and in his second edition, he relied almost entirely on Rask for Old English. His debt to Rask can be appreciated only by comparing his treatment of Old English in the two editions; the difference is very great. For example, in the first edition he declines disg, dceges, plural dcegas, without having observed the law of vowel change pointed out by Rask. There can be little doubt that the appearance of Rask's Old English grammar was the primary impetus for Grimm to recast his work from the beginning. To Rask also belongs the merit of having first distinctly formulated the laws of sound correspondence in the different languages, especially in the vowels (those more fleeting elements of speech previously ignored by etymologists).

The Grammar was continued in three volumes, treating principally derivation, composition and syntax, the last of which was unfinished. Grimm then began a third edition, of which only one part, comprising the vowels, appeared in 1840, his time being afterwards taken up mainly by the dictionary. The Grammar stands alone in the annals of science for its comprehensiveness, method and fullness of detail. Every law, every letter, every syllable of inflection in the different languages was illustrated by an almost exhaustive mass of material, and it has served as a model for all succeeding investigators. Diez's grammar of the Romance languages is founded entirely on its methods, which have also exerted a profound influence on the wider study of the Indo-European languages in general.

Grimm's Law, also known as 'Rask's - Grimm's Rule' is the first law in linguistics concerning a non-trivial sound change. It was a turning point in the development of linguistics, allowing the introduction of a rigorous methodology to historic linguistic research. It concerns the correspondence of consonants in the older Indo-European, and Low Saxon and High German languages, and was first fully stated by Grimm in the second edition of the first part of his grammar. The correspondence of single consonants had been more or less clearly recognized by several of his predecessors including Friedrich von Schlegel, Rasmus Christian Rask and Johan Ihre, the last having established a considerable number of literarum permutationes, such as b for f, with the examples bœra = ferre, befwer = fiber. Rask, in his essay on the origin of the Icelandic language, gave the same comparisons, with a few additions and corrections, and even the very same examples in most cases. As Grimm in the preface to his first edition expressly mentioned this essay of Rask, there is every probability that it inspired his own investigations. But there is a wide difference between the isolated permutations of his predecessors and his own comprehensive generalizations. The extension of the law to High German is entirely his own work, however.

The only fact that can be adduced in support of the assertion that Grimm wished to deprive Rask of his claims to priority is that he does not expressly mention Rask's results in his second edition. But this is part of the plan of his work, to refrain from all controversy or reference to the works of others. In his first edition he expressly calls attention to Rask's essay, and praises it most ungrudgingly. It is true that a certain bitterness of feeling afterwards sprang up between Grimm and Rask, but this may have well been the fault of the latter, who, impatient of contradiction and irritable in controversy, refused to play with the value of Grimm's views when they involved modification of his own.

Grimm's monumental German dictionary, (Deutsches Wörterbuch, or literally German Dictionary), remains a standard work of reference to the present day. The dictionary was undertaken on so large a scale as to make it impossible for him and his brother to complete it themselves. The dictionary, as far as it was worked on by Grimm himself, has been described as a collection of disconnected antiquarian essays of high value.

The first work Jakob Grimm published, Über den altdeutschen Meistergesang (1811), was of a purely literary character. Yet even in this essay Grimm showed that Minnesang and Meistersang were really one form of poetry, of which they merely represented different stages of development, and also announced his important discovery of the invariable division of the Lied into three strophic parts.

His text editions were mostly prepared in conjunction with his brother. In 1812 they published the two ancient fragments of the Hildebrandslied and the Weissenbrunner Gehet, Jakob having discovered what till then had never been suspected — namely the alliteration in these poems. However, Jakob had little taste for text editing, and, as he himself confessed, working on a critical text gave him little pleasure. He therefore left this department to others, especially Lachmann, who soon turned his brilliant critical genius, trained in the severe school of classical philology, to Old and Middle High German poetry and metre.

Both Brothers were attracted from the beginning by all national poetry, whether in the form of epics, ballads or popular tales. They published in 1816 – 1818 a collection of legends culled from diverse sources and published the two volume Deutsche Sagen (German Legends). At the same time they collected all the folktales they could find, partly from the mouths of the people, partly from manuscripts and books, and published in 1812 – 1815 the first edition of those Kinder- und Hausmärchen (Children's and Household Tales), which has carried the name of the brothers Grimm into every household of the western world. The closely related subject of the satirical beast epic of the Middle Ages also held great charm for Jakob Grimm, and he published an edition of the Rejnhart Fuchs in 1834. His first contribution to mythology was the first volume of an edition of the Eddaic songs, undertaken jointly with his brother, and published in 1815. However, this work was not followed by any others on the subject.

The first edition of his Deutsche Mythologie (German Mythology) appeared in 1835. This great work covered the whole range of the subject, tracing the mythology and superstitions of the old Teutons back to the very dawn of direct evidence, and following their evolution to modern day popular traditions, tales and expressions.

Jakob Grimm's work tied in strongly to his views on Germany and its culture. His work with fairy tales and his philological work dealt with German origins. He loved his people and wished for a united Germany. In the German revolution of 1848, he was given a chance to make these views known when he was elected to the Frankfurt National Parliament. The people of Germany had demanded a constitution, so the Parliament, formed of elected members from various German states, met to form one. Grimm was selected for the office in a large part because of his part in the University of Goettingen's refusal to swear to the king of Hanover expounded upon above. He then went to Frankfurt, where he did not play a very big part, but did make some speeches, which tended to stray into the realms of history and philology rather than whatever political question was at hand. Grimm was adamant on one subject, however; he wanted the duchy of Holstein of Denmark to be under German control. He talked passionately about this subject, which showed his fierce German nationalism. Grimm was not made to be a politician, and also soon realized that the National Assembly was not getting anywhere (it was eventually dissolved without establishing a constitution), and so asked to be released from his duties and returned with relief to his former studies. His political career did not bloom into anything great, but it does illustrate his characteristics - his nationalism and his moralism. He believed that good would triumph in the Parliament, and pushed for human rights legislation just as he wished for a unified Germany.