[1] Cowan transferred to Leeds during the 1962–63 season,[1] and scored 119-tries during his rugby league career.
Cowan played on the wing in Leeds' 2–18 defeat by Wakefield Trinity in the 1964–65 Yorkshire Cup Final during the 1964–65 season at Fartown Ground, Huddersfield on Saturday 31 October 1964, and played at centre in the 23–7 victory over Featherstone Rovers in the 1970–71 Yorkshire Cup Final during the 1970–71 season at Odsal Stadium, Bradford on Saturday 21 November 1970.
[citation needed] Cowan played at centre in Leeds' 9–5 victory over St. Helens in the 1970 BBC2 Floodlit Trophy Final during the 1970–71 season at Headingley, Leeds on Tuesday 15 December 1970.
Cowan played at centre (replaced by substitute Les Dyl) in Leeds' 7–24 defeat by Leigh in the 1970–71 Challenge Cup Final during the 1970–71 season at Wembley Stadium, London on Saturday 15 May 1971, in front of a crowd of 85,514.
[4] Cowan represented Other Nationalities (RL) while at Leeds, he was a substitute in the 2–19 defeat by St. Helens at Knowsley Road, St. Helens on Wednesday 27 January 1965, to mark the switching-on of new floodlights.