The second son of a Quaker merchant, he ran away from school to join the army, becoming an ensign in the 13th (Somersetshire) Light Infantry and serving under Sir Ralph Abercromby in Egypt in 1801. He accompanied the expedition (1809) to the Ionian Islands, where he met Theódoros Kolokotrónis and other Greeks in exile. Already sympathetic toward ideas of Greek independence, he afterward obtained permission to form two Greek regiments in English pay in order to give the Greeks military training, but the regiments were later disbanded. In vain, Church pleaded the Greek cause not only in London but at the Congress of Vienna (1814–15). He was knighted in 1822. In March 1827 he went to Greece to fight in the war of independence and in the following month was appointed commander in chief of the Greek forces. In 1829, after his campaigns in western Greece, Church resigned his command because he opposed the government of Count Kapodístrias. Later appointed confidential adviser to Sir Edmund (later Baron) Lyons, first British minister to Greece, he also played a conspicuous part in the revolution of 1843.