Oswald Spengler was born in 1880 in Blankenburg at the foot of the Harz mountains, the eldest of four children, and the only boy. His family was typically conservative German petit-bourgeoisie. His father, originally a mining technician, who came from a long line of mineworkers, was a post office bureaucrat. His childhood home was emotionally reserved, and the young Spengler turned to books and the great cultural personalities for succor. He suffered imperfect health, and was a lifelong sufferer of migraine headaches and an anxiety complex. At the age of ten, his family moved to the university city of Halle. Here Spengler received a classical education at the local Gymnasium (academically oriented secondary school), studying Greek, Latin, mathematics and natural sciences. Here, too, he developed his affinity for the arts — especially poetry, drama, and music — and came under the influence of the ideas of Goethe and Nietzsche. He even experimented with a few artistic creations, some of which still survive.

After his father's death in 1901, Spengler attended several universities (Munich, Berlin, and Halle) as a private scholar, taking courses in a wide range of subjects: history, philosophy, mathematics, natural science, literature, the classics, music, and fine arts. His private studies were undirected. In 1903, he failed his doctoral thesis on Heraclitus because of insufficient references, which effectively ended a chance for an academic career. In 1904, he received his Ph.D., and in 1905 suffered a nervous breakdown.

Scholars remark that his life seemed rather uneventful. He briefly served as a teacher in Saarbrücken and then Düsseldorf. From 1908 to 1911 he taught at a practical high school (Realgymnasium) in Hamburg, where he taught science, German history, and mathematics. In 1911, following his mother's death, he moved to Munich, where he would live until his death in 1936. He lived as a cloistered scholar, supported by his modest inheritance. Spengler lived on very limited means and was marked by loneliness. He owned no books, and took jobs as a tutor or writing for magazines to earn an additional income.

He began work on the first volume of Decline intending to focus on Germany within Europe at first, but was deeply affected by the Agadir Crisis, and widened the scope of his study. Spengler was inspired by Otto Seeck's work The Decline of Antiquity in naming his own effort. The book was completed in 1914, but publishing was delayed by World War I. During the war, his inheritance was largely useless because it was invested overseas; thus Spengler lived in genuine poverty for this period. After Decline came out in 1917, it was a wild success because of the perceived national humiliation of the Treaty of Versailles and later the economic depression around 1923 fueled by hyperinflation seemed to prove Spengler right (Spengler had in fact believed that Germany would win while he was writing the book). It comforted Germans because it seemingly rationalized their downfall as part of larger world-historical processes. It was widely successful outside of Germany as well, and by 1919 had been translated into several other languages. He rejected a subsequent offer to become Professor of Philosophy at the University of Göttingen, saying he needed time to focus on writing.

The book was widely discussed, even by those who had not read it. Historians took umbrage at an amateur effort by an untrained author and his unapologetically non-scientific approach. Thomas Mann compared reading Spengler's book to reading Schopenhauer for the first time. Academics gave it a mixed reception. Max Weber described Spengler as a "very ingenious and learned dilettante" while Karl Popper described the thesis as "pointless". The great historian of antiquity Eduard Meyer thought highly of Spengler, although he also had some criticisms of him. Spengler's obscurity, intuitionalism, and mysticism were easy targets, especially for the Positivists and neo-Kantians who saw no meaning in history. The critic and æsthete Count Harry Graf Kessler thought him unoriginal and rather inane, especially with regards to his work on Nietzsche. Ludwig Wittgenstein, however, shared Spengler's cultural pessimism. Spengler's work became an important foundation for the social cycle theory.

In the second volume, published in 1920, Spengler argued that German socialism was different from Marxism, and was in fact compatible with traditional German conservatism. In 1924, following the social-economic upheaval and inflation, Spengler entered politics in an effort to bring Reichswehr general Hans von Seeckt to power as the country's leader. The effort failed and Spengler proved ineffective in practical politics. In 1931, he published Man and Technics, which warned against the dangers of technology and industrialism to culture. He especially pointed to the tendency of Western technology to spread to hostile "Colored races" that would then use the weapons against the West. It was poorly received because of its anti-industrialism. This book contains the well-known Spengler quote, "Optimism is cowardice."

Despite voting for Hitler over Hindenburg in 1932, Spengler found the Führer vulgar. He met Hitler in 1933 and after a lengthy discussion remained unimpressed, saying that Germany didn't need a "heroic tenor (Heldentenor: one of several conventional tenor classifications) but a real hero ("Held")." He publicly quarreled with Alfred Rosenberg, and his pessimism and remarks about the Führer resulted in isolation and public silence. He further rejected offers from Joseph Goebbels to give public speeches. However, Spengler did become a member of the Academy of Germany in the course of the year.

The Hour of Decision, published in 1934, was a bestseller, but was later banned by the Nazis for its critiques of National Socialism. Spengler's criticisms of liberalism were welcomed by the Nazis, but Spengler disagreed with their biological ideology and anti-Semitism. While racial mysticism played a key role in his own worldview, Spengler had always been an outspoken critic of the pseudoscientific racial theories professed by the Nazis and many others in his time, and was not inclined to change his views upon Hitler's rise to power. Although himself a German nationalist, Spengler also viewed the Nazis as too narrowly German, and not occidental enough to lead the fight against other peoples. The book also warned of a coming world war in which Western Civilization risked being destroyed, and was widely distributed abroad before eventually being banned in Germany. A Time review of The Hour of Decision noted his international popularity as a polemicist, observing that "When Oswald Spengler speaks, many a Western Worldling stops to listen." The review recommended the book for "readers who enjoy vigorous writing", who "will be glad to be rubbed the wrong way by Spengler's harsh aphorisms" and his pessimistic predictions.

He spent his final years in Munich, listening to Beethoven, reading Molière and Shakespeare, buying several thousand books, and collecting ancient Turkish, Persian and Hindu weapons. He made occasional trips to the Harz mountains, and to Italy. Shortly before his death, in a letter to a friend, he remarked that "the German Reich in ten years will probably no longer exist". He died of a heart attack on May 8, 1936, three weeks before his 56th birthday and exactly nine years before the fall of the Third Reich.

Although he was highly influential and internationally popular during the interwar period, Oswald Spengler's work fell into intellectual disrepute and obscurity following World War II. One of the main reasons he was widely disliked or ignored is that he had been a leading opponent of the Weimar Republic. Only recently has interest in Spengler been rekindled. His concepts, opinions, theories, and predictions remain controversial among admirers and detractors alike.