Julien Green was born to American parents in Paris, a descendant on his mother's side of a Confederate Senator, Julian Hartridge (1829–1879), who later served as a Democratic Representative from Georgia to the US Congress, and who was Julien Green's namesake. (Green was christened "Julian", the spelling was changed by his French publisher in the 1920s to "Julien.")

Born into a Protestant family, he converted to Catholicism in 1916. The next year, at the age of 16, he volunteered his services as an ambulanceman in the American Field Service. When his age was discovered his enlistment was annulled. He immediately signed up with an ambulance unit of the American Red Cross, and when that six-month term of service ended in 1918, he enlisted in the French Army, in which he served as a second lieutenant of artillery until 1919. He was educated at the University of Virginia in the United States from 1919-22. His career as one of the major figures of French literature in the 20th century started soon after his return from the United States. In July 1940, after France's defeat, he went back to America. In 1942, he was mobilized and sent to New York to work at the United States Office of War Information. From there, for almost a year, five times a week, he would address France as part of the radio broadcasts of Voice of America, working inter alia with André Breton and Yul Brynner. Green went back to France right after the end of World War II. A devout Catholic, most of his books focused on the ideas of faith and religion as well as hypocrisy. Several of his books dealt with the southern United States, and he strongly identified with the fate of the Confederacy, characterizing himself throughout his life as a "Sudiste." He inherited this version of patriotism from his mother, who came from a distinguished southern family. Some years before Julien's birth, when Julien's father was offered a choice of posts (with his bank) in either Germany or France, Julien's mother urged the choice of France on the grounds that the French were "also a proud people, recently defeated in war, and we shall understand one another." (The reference was to France's 1871 defeat in the Franco-Prussian War).

In France, both during his life and today, Julien Green's fame rests principally not on his novels, but on his journals, published in ten volumes, and spanning the years 1926-1976. These avidly-read and well-known volumes provide a chronicle of his literary and religious life, and a unique window on the artistic and literary scene in Paris over a span of half a century. Green's style, austere and employing to great effect the "passé simple", a literary tense nearly abandoned by many of his French contemporaries, found favor with the Académie française, a fact mentioned in his election to that august body. Green resigned from the Académie shortly before his death, citing his American heritage and loyalties.

While Green wrote primarily in the French language, he also wrote in English, being entirely bilingual. He translated some of his own works from French to English (sometimes with the help of his sister, Anne). A collection of some of his translations is published in Le langage et son double, with a side-by-side French-English format, facilitating direct comparison. Despite his being bilingual, Green's texts remain largely unknown in the English-speaking world.

Thus far three of his books have been turned into films: Léviathan (1962), for which he wrote the screenplay, is the most famous. Adrienne Mesurat (1953), and La Dame de pique (1965) were also adapted to film.

He died in Paris and was buried in the parish church of Klagenfurt, Austria. Apparently, Green was greatly moved by a statue of the Virgin Mary during a visit there in 1990. Subsequently, he expressed a desire to be buried in one of the church's chapels.

Julien Green adopted gay fiction writer Éric Jourdan as his son. According to Jourdan, Green decided to move to a house belonging to Caterina Sforza in Forlì, Italy, in 1994. However, Green did not move to this house because his health was failing.

Green was the first non-French national to be elected to the Académie française. Fittingly, he succeeded François Mauriac, taking chair number 22 on June 3, 1971. It was commonly believed he had dual citizenship, but in fact, although born in Paris and writing almost exclusively in the French language, he had never become a French citizen. President Georges Pompidou offered him French citizenship in 1972 following Green's election to the Académie, but - faithful to the patriotic, "sudiste" spirit his mother inspired in him - Green refused it.

He revealed in his fiction and non-fiction that he was homosexual. However, he was capable of writing about women and heterosexuality as exemplified in his surrealist novel Minuit (Midnight) that explores the sexual awakening of an adolescent young girl.