In his long career he has represented the New Zealand national rugby league team within rugby league and New Zealand Māori sides and has played for the Leeds Rhinos, Bradford Northern, Warrington Wolves, Castleford Tigers and Halifax.
[6] Gary Mercer played at centre in Warrington's 14-36 defeat by Wigan in the 1990 Challenge Cup Final during the 1989–90 season at Wembley Stadium, London on Saturday 28 April 1990, in front of a crowd of 77,729.
[7] Gary Mercer played fullback in Bradford Northern's 12-12 draw with Castleford in the 1987 Yorkshire Cup Final during the 1987–88 season at Headingley, Leeds on Saturday 17 October 1987, and played at centre in the 11-2 victory over Castleford in the 1987 Yorkshire Cup Final replay during the 1987–88 season at Elland Road, Leeds on Saturday 31 October 1987.
Gary Mercer played at second-row in Warrington's 12-2 victory over Bradford Northern in the 1990–91 Regal Trophy Final during the 1990–91 season at Headingley, Leeds on Saturday 12 January 1991.
[18] Founding member of the FUDs society, with co-members Toby Harris and Nathan Rainbow.