His parents were Louis and Marie Lucie, of Pomonti. He had two brothers, Thomas and Anthony. Thomas was killed in the Battle of Wagram. Anthony was mute from birth. Giuseppe spent his childhood and adolescence as a shepherd. In 1808 he joined a Corsican regiment and was sent to Naples, then to Russia to fight in the Napoleonic Wars. In 1812 he held the rank of sergeant. He was discharged from the army in 1814 and returned to Corsica. In September 1815, he was one of around 1000 followers who joined former King of Naples Joachim Murat in an attempt to regain his kingdom, this ended a month later with Murat's capture and execution by forces of Ferdinand IV of Naples. According to Harsin, Fieschi escaped execution and was deported to France, where he was eventually sentenced in 1816 to 10 years in jail for the theft of a steer. There he met a female inmate, Laurence Petit. Upon his release in 1826 he moved to Lyon, Petit's hometown.