Gary was born Roman Kacew (Yiddish: קצב, Russian: Касев), born in Vilnius to a family of Lithuanian Jews. Gary changed his name to Romain Gary when he escaped occupied France to fight with Great Britain against Germany in World War II. His father, Arieh-Leib Kacew, abandoned his family in 1925 and remarried. Although Gary never liked to discuss his father's identity, his recent biographer Dominique Bona theorizes convincingly that he was the son of the actor Ivan Mozzhukhin. After his father left Gary was raised by his Jewish mother, Nina Owczinska. When Gary was fourteen, he and his mother moved to Nice, France. In his books and interviews, he presented many different versions of his father's origin, parents, occupation and childhood.

Gary's first wife was the British writer, journalist, and Vogue editor Lesley Blanch (author of The Wilder Shores of Love). They married in 1944 and divorced in 1961. From 1962 to 1970, Gary was married to American actress Jean Seberg, with whom he had a son, Alexandre Diego Gary. According to Diego Gary, he was a distant presence as a father; "Even when he was around, my father wasn't there. Obsessed with his work, he used to greet me, but he was elsewhere."

Gary studied law, first in Aix-en-Provence and then in Paris. He learned to pilot an aircraft in the French Air Force in Salon-de-Provence and in Avord Air Base, near Bourges.

Following the Nazi occupation of France in World War II, he fled to England and under Charles de Gaulle served with the Free French Forces in Europe and North Africa. As a pilot, he took part in over 25 successful sorties, logging over 65 hours of air time.

He was greatly decorated for his bravery in the war, receiving many medals and honours among which Compagnon de la Libération and commander of the Légion d'honneur.

After the war, he worked in the French diplomatic service and in 1945 published his first novel.

In 1952, he became secretary of the French Delegation to the United Nations in New York, and later in London (in 1955).

In 1956, he became Consul General of France in Los Angeles.

Gary would become one of France's most popular and prolific writers, authoring more than thirty novels, essays and memoirs, some of which he wrote under a pseudonym.

He is the only person to win the Prix Goncourt twice. This prize for French language literature is awarded only once to an author. Gary, who had already received the prize in 1956 for Les racines du ciel, published La vie devant soi under the pseudonym of Émile Ajar in 1975. The Académie Goncourt awarded the prize to the author of this book without knowing his real identity. A period of literary intrigue followed. Gary's cousin's son Paul Pavlowitch posed as the author for a time. Gary later revealed the truth in his posthumous book Vie et mort d'Émile Ajar