On either 30 June or 1 July 1559, during a jousting match to celebrate the Peace of Cateau Cambrésis between Henry II and his longtime Habsburg enemies, a splinter of wood from Montgomery's shattered lance pierced Henry's eye and entered his brain, mortally injuring him. From his deathbed Henry absolved Montgomery of any blame, but, finding himself disgraced, Montgomery retreated to his estates in Normandy. There he studied theology and converted to Protestantism, making him an enemy of the state.
The fatal tournament between Henry II
and Montgomery (Lord of Lorges).
In 1562, Montgomery allied himself with another Protestant convert, Louis I de Bourbon, prince de Condé. He was one of the few refugees to survive the St. Bartholomew's Day massacre after a wounded Huguenot swam across the Seine to warn him that rioting had begun. He took control of Bourges and during September and October defended Rouen from the Royal Army. A price was put on his head, but he managed to escape to England. The queen mother, Catherine de' Medici, asked Queen Elizabeth I for his , but Elizabeth refused.
Montgomery returned to France with a fleet in an attempt to relieve the Siege of La Rochelle in 1573. The following year he attempted an insurrection in Normandy, but was captured, taken to Paris, and sentenced to death. On 26 June 1574, as he was about to be beheaded, Montgomery was informed that a royal edict had proclaimed that his property would be confiscated and his children deprived of their titles.
A freely adapted version of Montgomery's life is told in Alexandre Dumas' novel The Two Dianas.