Mavromichalis took part in the Greek War of Independence fighting against the forces of Dramali Pasha. Earlier, after the Siege of Tripolis, he was put in charge of guarding the harem of Hursid Pasha until the Turks ransomed them.
In October of 1822, Georgios Mavromichalis, along with Bishop Germanos of Patra, left on a mission to Italy to seek help from the Pope for the Greek Revolution. The two men got as far as Verona but were refused passage to Rome and had to return to Greece without accomplishing their purpose.
In 1826, Ibrahim Pasha sought to invade Mani. The Egyptian pasha sent word to the Maniotes that if they did not surrender, he wouldn't leave any trace of any house anywhere in Mani. Georgios Mavromichalis sent him the reply: "Come on over with as many men as you can muster. We'll be expecting you". Led by Mavromichalis, the Maniotes defeated Ibrahim Pasha setting up the stage for the Battle of Navarino.
After Greek independence was achieved, Mavromichalis' father was arrested and charged with high treason on the orders of governor Ioannis Kapodistrias. Georgios and his uncle Konstantinos Mavromichalis decided to take revenge.
On September 27, 1831 the two men awaited the governor outside St Spyridon church in Nafplio. As his uncle stabbed Kapodistrias, Georgios shot the governor. An angry Nafplio mob captured Konstantinos and lynched him mercilessly before he was shot dead by General Christos Fotomaras. Georgios managed to escape to the French Embassy where he asked for asylum. His request was refused and the French handed him back to the Greek authorities after receiving guarantees that he would not be lynched.