Fahie was born in Saint Kitts, one of the Leeward Islands, the son of a judge and part of a prominent family of Irish settlers. In 1777, aged 14, he was sent to serve in the Royal Navy during the American Revolutionary War, joining under Captain John Colpoys. Within a year he had moved to , later transferring again to Sir George Rodney's flagship . On "Sandwich" he was engaged in the Battle of Cape St Vincent off Southern Spain in 1780. Appointed lieutenant in the aftermath of the action, Fahie returned to the West Indies on , serving at the Battle of Martinique and the Battle of Saint Kitts. Due to his knowledge of the island, Fahie was selected to take messages to the besieged garrison on Saint Kitts but was briefly captured in the attempt, before being released. He later served at the Battle of the Saintes.