Henry married Lucy Flucker (1756–1824), the daughter of Boston Loyalists, on June 16, 1774, despite opposition from her father that was due to their differing political views. Lucy's brother served in the British Army, and her family attempted to lure Knox to service there. Despite long separations due to his military service, the couple were devoted to one another for the rest of his life, and carried on an extensive correspondence. After the couple fled Boston in 1775, she remained essentially homeless until the British evacuated the city in March 1776. Even afterward, she often traveled to visit Knox in the field. Her parents left, never to return, with the British during their withdrawal from Boston after the Continental Army fortified Dorchester Heights, a success that hinged upon Knox's Ticonderoga expedition.