Alberico Gentili was born into a noble family in the town of San Ginesio in what is now Marche, in Italy. He studied law at the university of Perugia and graduated doctor of law in 1572. He was commissioned to prepare a revised version of the statutory laws of his home town, a task which he completed in 1577. Two years later, together with his father Matteo Gentili (1517 — 1602), a renowned medical doctor, and one of his brothers, Scipione Gentili, he had to flee from Italy because of their Protestant beliefs. The three first went to Ljubljana, Slovenia, the capital of the duchy of Carniola. From there, Alberico went on to the German university towns of Tübingen and Heidelberg. In 1580 he arrived in England and was appointed regius professor of civil law at Oxford University by the then Chancellor, Robert Dudley, 1st Earl of Leicester. After a short stay in Wittenberg, Germany, in 1586, he returned to Oxford.

Gentili held the regius professorship until his death, but he turned more and more to practical work in London from about 1590. He practised in the High Court of Admiralty, where the continental civil law rather than the English common law was applied. In 1600 Gentili was honorifically admitted to Gray's Inn. From 1605 to 1608 he served as a standing advocate to the Spanish embassy. He died in London and was buried in the Church of St. Helen Bishopsgate in the City of London.

His son was Robert Gentilis, who graduated from Oxford University at the age of 12 and was made a Fellow of All Souls College Oxford at the age of 17 (below the minimum age of 18) through his father's influence.

Alberico Gentili wrote more than twenty books not only on law, but also on theological and literary subjects. Only his most influential legal works are mentioned here.

In 1582, Gentili published De Juris Interpretibus Dialogi Sex. This book shows Gentili as a staunch supporter of the bartolist method and an opponent of the French humanist jurists like Jacques Cujas, who applied philological methods to the sources of Roman law.

Gentili's first book on issues of international law was De Legationibus Libri Tres, published in 1582. It was occasioned by a case on which Gentili's counsel was sought. In 1584 Gentili and Jean Hotman (1552 — 1636) were asked by the government to advise on the treatment of Spanish ambassador Bernardino de Mendoza (about 1540 – 1604), who had been implicated in the so-called Throckmorton plot against Queen Elizabeth I. Hotman was the son of the French law professor François Hotman (1524 – 1590) and — like Gentili — a lawyer trained on the continent who had come to England for religious reasons. He was in the service of the Earl of Leicester. Hotman, too, later published a book on diplomacy, L'ambassadeur, first published in Paris in 1603. Both Gentili and Hotman recommended that the ambassador only be expelled from England.

In 1589 Gentli first published De Jure Belli Commentationes Tres. An enhanced edition appeared under the title De Jure Belli Libri Tres. This is considered his principal work and a classic of public international law. The book is not only praised for its modernity and its skillful use of civil law concepts, but also for its closeness to the actual practice of international law.

After his death, Alberico Gentili's brother Scipione, who had become a professor of law at Altdorf, published a collection of notes on cases Alberico had worked on as an advocate for the Spanish embassy. The book bears the title Hispanicae Advocationis Libri Duo and appeared in 1613.