Sassi was born in French-ruled Tunis, from a family of Corsican origin. His grandfather had been a French soldier during the Tonkin Campaign. The young Sassi aspired to be a championship swimmer.
During the Battle of Dien Bien Phu in April 1954 Jean Sassi led Mèo partisans (GCMA) in Operation Condor, also known as Operation D. He also participated in the French war in Algeria. Promoted to Commandant (equivalent to Major) on February 2, 1960 but was sent home for serious medical issues. He left the army at the age of 54 with the rank of colonel and then joined the personnel department of Citroën.