 In 1971, he signed his professional contract at the club, and made his debut on 4 April 1972 against Preston North End in a 3–1 win.His first goal in a Sunderland shirt came against Middlesbrough on 17 February 1973, when Sunderland won 4–0.Horswill played a role in Sunderland's run in their 1972–73 FA Cup run, as he played in every game, and also featured in the final of the tournament, where Sunderland, a Second Division side at the time, beat Leeds United 1–0.