Horn was born on 15 October 1915. He joined the Danish army and trained as a pilot, graduating as a qualified pilot in 1937 (certificate 214/37). On 9 October 1938 he was made Sekondløjtnant (2nd Lieutenant) and at the outbreak of war, was a Lieutenant in the reserve.
On 8 July 1941 Danish commissioned officers of the armed forces are officially allowed to volunteer for the Freikorps Denmark for service in the Finnish Air Force on the Eastern Front. One of the eight volunteers was Horn. In the summer of 1941 the Finnish Air Force no longer needed foreign volunteers, so the Danes were transferred to the Luftwaffe.
Leutnant Horn remained in Luftwaffe service until the end of the war, as a fighter pilot with 1./JG 51 from 1941 to 1943. He survived the war.
He was credited with 10 or 11 victories during the war, mostly on the Eastern Front. During his Luftwaffe service Horn was decorated with the Iron Cross 1st and 2nd Class.