He was born in Juiz de Fora, in the state of Minas Gerais, but he has lived most of his life in Rio de Janeiro. In 1952, he started his career as a low-level cop and, later became a police commissioner, one of the highest ranks in the civil police of Brazil. Following the steps of American novelist Thomas Pynchon, a close friend of Fonseca, he refuses to give interviews and feels strongly about maintaining his privacy.
His writing is pretty dark and gritty, filled with violence and sexual content, and it usually happens in an urban setting. He says that a writer should have the courage to show what most people are afraid to say. His work is considered groundbreaking in Brazilian literature[by whom?], up until then mostly focused on rural settings and usually treating cities with less interest[dubious– discuss]. Almost all Brazilian contemporary writers acknowledge Fonseca's importance. Authors from the rising generation of Brazilian writers, such as Patrícia Melo or Luis Ruffato, have stated that Fonseca's writing has influenced their work.
He started his career by writing short stories, considered by some critics as his strongest literary creations. His first popular novel was A Grande Arte (High Art), but "Agosto" is usually considered his best work.