Fesch's father was a wealthy banker of Belgian origin, an artist and atheist, distant from his son and unfaithful to his wife, whom he ultimately divorced. Jacques was an idler; brought up a Roman Catholic, he abandoned religion by the age of 17. At 21, he married his pregnant girlfriend Pierrette in a civil ceremony. He gave up a position at his father's bank, lived the life of a playboy, left his wife and their daughter, and fathered an illegitimate son with another woman. Disillusioned with his life, he dreamed of escaping to sail around the South Pacific Ocean, but his parents refused to pay for a boat.

On 24 February 1954, to fund the purchase of a boat, he went to rob Alexandre Sylberstein, a money changer, of gold coins. Sylberstein was struck but not unconscious, and raised the alarm. Fesch fled, losing his glasses, and shot wildly at Jean Vergne, a pursuing police officer, killing him. Minutes later he was arrested. Murdering a police officer was a heinous crime and public opinion, inflamed by newspaper reports, was strongly in favour of his execution. The Cour d'assises of Paris condemned him to death on 6 April 1957.

At first Fesch was indifferent to his plight and mocked his lawyer's Catholic faith. However, after a year in prison, he experienced a profound mystical conversion, became very pious, and bitterly regretted his crime. He corresponded regularly with his family, notably his brother and stepmother, and kept a spiritual journal. He accepted his punishment serenely and was reconciled to his wife the night before his execution. His last journal entry was "In five hours, I will see Jesus!". An appeal for clemency to President René Coty failed, and he was guillotined.

After his death his wife and daughter honoured his memory as an example of redemption. At first he was excoriated by the public, but with the work of Soeur Véronique, a Carmelite nun, and Fathers Augustin-Michel Lemonnier, the family effected publication of his writings, and from the 1970s these served as an inspiration to many.

On September 21, 1987 the Archbishop of Paris, Cardinal Jean-Marie Lustiger, opened a diocesan inquiry into his life; the cause for his beatification was formally opened in 1993. This has proved controversial, with those who feel his early crimes make him unfitting as a role model opposed to those who emphasize the hope of his final conversion.