Oscar Milosz was born in Čareja (Chereya), Minsk Governorate, Russian Empire, now in modern-day Belarus, where he also spent his childhood. His father, Vladislas de Lubicz Milosz, was ethnically Lithuanian, nominally Catholic, and for a time an officer in the imperial Russian army. His mother, Marie Rosalie Rosenthal, was Jewish, the daughter of a Hebrew professor at the University of Warsaw. The family spoke Polish at home. Oscar was baptized a Catholic on 2 July 1886, at St. Alexander’s Church in Warsaw. In 1889, when he was 12, his parents placed him at the Lycée Janson de Sailly in Paris. He began writing poems in 1894 and started to frequent artistic circles, meeting Oscar Wilde and Jean Moréas. After finishing at the Lycée, he enrolled at the École des langues orientales, where he studied Syriac and Hebrew.

His first book of verse, Le Poème des Décadences, appeared in 1899. In the first years of the twentieth century, Milosz travelled widely in Europe and North Africa and explored many foreign literatures. A European poet of the French language, Milosz was an excellent linguist and was fluent in French, Polish, Russian, English, German, Italian, and Spanish as well as being able to read Latin and Hebrew. Later in life, he would also learn Lithuanian and Basque.

Milosz published his second poetry collection, the more accomplished Les Sept Solitudes, in 1906. He then entered into a phase of literary experimentation during which he tried his hand at a novel, L’Amoureuse Initiation, published in 1910, and three “mystery dramas,” the most popular of these plays being Miguel Mañara (1913), a reworking of the Don Juan myth. During this time he also composed his third poetry collection, Les Éléments (1911).

On 14 December 1914, while saying his prayers at the end of an evening of intensive reading of the Bible and Emanuel Swedenborg, Milosz experienced an illumination that led him to proclaim the next day to a friend: “I have seen the spiritual sun.” Influenced by this vision, his poetry became more profound. He began to study the Kabbalah, and Renaissance and Baroque alchemists and thinkers like Paracelsus and Jacob Boehme. After 1916, the development of his metaphysics became his major poetic preoccupation. He began to develop a literary cosmogonic system in the tradition of Lucretius, William Blake, and Edgar Allan Poe and exposed it for the first time in the essay Épitre à Storge, published in La Revue de Hollande in 1917. In the early 1920s, Milosz convinced himself that his poetic cosmogony was supported by Einstein’s theory of relativity, still a subject of debate. During this period, after a flirtation with “occult” reading and friends, like the numerologist René Schwaller de Lubicz, Milosz turned his back on these currents of thought and began to study medieval science and thinkers like the English scholastic Robert Grosseteste. Finally, in 1927 he took a Father Confessor and became a practicing Roman Catholic, which he remained for the last twelve years of his life.

In 1916, during World War I, Milosz was conscripted to the Russian division of the French army and was assigned to the press corps. After the Russian Revolution of October 1917, Čareja was seized by the Soviets. Suddenly, access to his family fortune was cut off and Milosz needed to earn a living. Around this time he learned about the growing movement for Lithuanian independence. By the end of the war when both Lithuania and Poland were effectively independent again, Milosz chose to identify with Lithuania – even though he did not yet speak Lithuanian — because he believed that it had been the original homeland of his ancestors in and prior to the 13th century. In 1920 when France recognized the independence of Lithuania, he was officially appointed Chargé d’Affaires for the new state. Milosz’s diplomatic career remains one of the more fascinating aspects of his legacy; his articles and correspondence in the service of the reborn Lithuanian state show a high level of nuance and rigor. In 1931 he became a French citizen and was awarded the Légion d’honneur.

Milosz was largely neglected during his lifetime. He has increasingly, however, come to be considered an important figure in French poetry. In a 1926 letter to James Chouvet, he writes:

In 1939, shortly after retiring from his diplomatic post and ill with cancer, he died of a heart attack in a house he had recently purchased in Fontainebleau. He is buried in the cemetery at Fontainebleau. Every year, around the time of his birthday on May 28, a group of admirers, Les Amis de Milosz, commemorate his life and work in a ceremony at the grave site.