Born in Bo'ness, Scotland, he started his professional career in the Football League, helping Watford to the Third Division title, and then spending three seasons at Sheffield United.
However, Scullion did not play a league game for Charlton, and in February 1966 he was signed by Watford manager Ken Furphy in a deal that saw Cliff Holton move in the opposite direction.
[2][3] On 25 January 1969, Scullion scored the opening goal in a 1–1 draw against Manchester United in the Fourth Round of the FA Cup at Old Trafford.
[7] During his two and a half years in Sheffield, Scullion played 57 league games, scoring 7 goals, before being sold back to Watford for £15,000 in December 1973.
In his second season, he scored 10 times in 24 appearances, with Tampa winning the Eastern Division Atlantic Conference, and Scullion again being named in the All-star second team.
Playing alongside Pelé, Bobby Moore and Giorgio Chinaglia, Scullion scored Team America's only goal of the tournament, in a 3–1 defeat to England.