Bost was born the youngest of ten children on 6 May 1916 in Le Havre, Normandy, France to Pastor Charles Bost.[3][4][5] One of his siblings, older brother Pierre, was a screenwriter and author[4] and journalist Serge Lafaurie was his nephew.[citation needed] Bost was known as "little Bost" in the 1930s because Pierre had already made a name for himself.[2] One of Bost's teachers at Lycée du Havre was Jean-Paul Sartre, with who he became lifelong friends.[6][1]
Bost was a second-class infantryman in the French Army during World War II until being injured in May/June 1940.[1][7][2][8] He and Simone de Beauvoir exchanged a number of letters while he was deployed; their correspondence would later be published by Beauvoir's daughter Sylvie under the title Correspondance croisée.[9] Following his service, for which he received a Croix de Guerre, he worked as a war correspondent for Combat.[5][10] Bost came across Dachau just hours after the American troops during one of his assignments.[5]
Bost wrote for L'Express after Combat but left in 1964 to co-found L'Obs with Jean Daniel, Serge Lafaurie, K. S. Karol, and André Gorz.[11][12][5] During his career, he also wrote for the satire weekly Le Canard enchaîné and Sartre's Les Temps modernes.[13][14][15][2] At times, he published under the pseudonym Claude Tartare.[citation needed]Le Dernier des métiers (1946) was the only book Bost wrote and published during his life.[2][16] The character Boris in Sartre's Les Chemins de la liberte is based on Bost; Beauvoir also mentioned him in La Force de l'age.[2]
Bost and Beauvoir began their affair in 1939 and he was described as her second love only to Sartre.[6] He married Olga Kosakiewicz, one of the many women involved in a ménage à trois with Beauvoir and Sartre, but Bost and Beauvoir continued their affair in secret.[6][20][1] Early in their friendship, Bost is thought to have had sex with Natalie Sorokin at the suggestion of Beauvoir.[21] Bost and Olga had at least one child, Bernard Edouard.[22] Olga died in 1985.[23]
Bost died from cancer on 21 September 1990 in Paris.[11][2]