A Wales and Great Britain international representative prop, "Big Jim" as he was known, played club rugby in England with Halifax, Salford, Bradford Northern, Widnes (two spells) (with whom he won two Challenge Cups) and Workington Town, and also in Australia for North Sydney.
Mills played left-prop in Widnes' 0–5 defeat by Leigh in the 1972 BBC2 Floodlit Trophy Final during the 1972–73 season at Central Park, Wigan on Tuesday 19 December 1972.
Mills played left-prop in Widnes' 2–3 defeat by Bradford Northern in the 1974–75 Player's No.6 Trophy Final at Wilderspool Stadium, Warrington on Saturday 25 January 1975.
After a strong display in Wales' victory over England in the 1975 World Series in Brisbane, Sydney's Manly-Warringah and Canterbury-Bankstown clubs were keen to get the prop's signature.
He played left-prop in Workington Town's 11–16 defeat by Widnes in the 1976 Lancashire Cup Final during the 1976–77 season at Central Park, Wigan on Saturday 30 October 1976, and played left-prop in Widnes' 15–13 victory over Workington Town in the 1978 Lancashire Cup Final during the 1978–79 season at Central Park, Wigan on Saturday 7 October 1978.
Mills played prop in the 12–3 victory over Wakefield Trinity in the 1979 Challenge Cup Final during the 1978–79 season at Wembley Stadium, London on Saturday 5 May 1979.