Seed spent the 1914–15 season playing in Sunderland reserves, he scored plenty of goals as the team lifted the Durham Senior Cup. Official League football was suspended at the end of that season because of the outbreak of World War I. At the end of the season, the 20-year-old Seed joined the Army Cyclist Corps. In the summer of 1916, he was drafted to France with the 8th battalion of the West Yorkshire Regiment. In July 1917 Seed was gassed when a German aeroplane dropped mustard gas bombs over Nieuwpoort, Belgium. Seed was with other soldiers sheltering in the basement of a bombed out building when the gas seeped in. Over fifty of his comrades died in the incident. He was sent to England to convalesce and returned to France in August 1918, until being evacuated after being gassed in Valenciennes, France two months later. With the cessation of hostilities, Seed played a Victory League match for Sunderland against Durham City in 1918, however his lungs were weak and he had a poor game. On the strength of that performance the Sunderland directors decided that Seed's wartime experience had finished him as a footballer and let him go, although they didn't put him on the transfer list as they thought his lungs were too damaged. Seed never played an official first team game for Sunderland. He was discharged from the army in March 1919.