Born to Fabien Delon, a small cinema owner from Corsica, and Edith Delon, a pharmacist from Algeria. In 1939, his parents divorced and he was placed in a Catholic boarding school in Isy de Mulino. He was punished six times for discipline and mistakes at school. When his mother remarried to a merchant, Paul Bulon, Allen took up the profession of a sausage maker and helped his stepfather.

Called up at the age of seventeen, Allen served at the Saigon Arsenal in Vietnam, and took part in the Indochina War. On his return from service in 1956, he met Jean-Claude Brialy, who invited him to the Cannes Film Festival, where his appearance, physical features and charisma of communication caught the eye of many film actors. Already in 1957, Joe's small role in his first film, Quand la femme s'en mêle (When a Woman Intervenes), brought him a success that would follow him throughout his 87 films. At the age of 23, he was already being compared to French film stars such as Gerard Philippe and Jean Marais, as well as to the American actor James Dean.

In 1958, during the production of Christine, he met Romi Schneider, and their engagement on 22 March 1959 was hailed by the media as a celebration of youth, beauty and success. However, after being engaged for six years, they never became husband and wife. In 1964, his first wife was the actress Natali Kanov, with whom he had a son, Anthony, on 30 September. After four years of marriage, they divorced. In 1968, during the making of the film "Jeff", he met the actress Mireille Darc. Their partnership lasted for 15 years, until 1982. In 1987, Allen met the Dutch model Rosalie van Breemen. In 1990, their daughter Anushka was born, and in 1994 their son Allen-Fabien. The couple divorced in 2002.

In 1964 he founded the film company Adel, and since 1968 he has worked as a producer. He has mainly acted in adventure films and detective stories. In 1981 he made his first feature film, Pour la peau d'un flic (For a policeman's head).

In 1985 he became a Swiss citizen and settled near Lake Leman.