When his father was captured, placed in prison and charged with high treason on the order of Governor of Greece John Capodistria, Georgios and his uncle Kostantinos Mavromichalis decided to take revenge. On 9 October 1831, the duo decided to assassinate the Governor as he entered St Spyridon church in Nauplio. Konstadinos firstly attempted to shoot Capodistrias, but he missed, which led him to stabbing him in the stomach and Georgios stabbed the president through the heart. As they were escaping Konstantinos was shot. Georgios managed to escape and hide in the French Embassy; after a few days he surrendered to the Greek authorities. He was sentenced to death by a court-martial and was executed by firing squad.