The son of Adolph and Selma (Koch) Boysen, Harold Boysen joined the Royal Flying Corps in Canada in 1917. Following flight training, he was assigned to 66 Squadron on the Western Front where he flew the Sopwith Pup. Re-equipped with Sopwith Camels, 66 Squadron was transferred to the Italian front in November 1917. Boysen scored his first victory on 8 December 1917, shooting down an Albatros D.V near Valstagna. On 28 January 1918, he was badly injured in a crash while attempting to land his Camel in the fog. When he recovered, he returned to duty and scored four more victories in May 1918.