Brett Mullins made his first grade début for Canberra from the substitution bench in a 48–0 rout of the South Sydney Rabbitohs in 1990.
Mullins played on the wing, in the centres and also filled in at fullback during 1991, but an injury suffered in the Minor Preliminary Semi-final against Western Suburbs saw him miss a place in the Raiders Grand Final side that went down 12-19 to the Penrith Panthers.
Mullins continued playing in the three-quarters before securing the Raiders fullback spot following the retirement of Gary Belcher at the end of the 1993.
During the 1994 NSWRL season, Mullins overcame several niggling injuries to play in the Raiders 36–12 Grand Final victory over Canterbury-Bankstown.
"[4] In 2000, in his final year with the club, Mullins emulated his father by surpassing the 100 try scoring mark in first grade rugby league.
[6] Mullins returned to Australia for the 2002 NRL season having signed with the Sydney Roosters (his father's former team when known as Eastern Suburbs) under the coaching of former Canberra, New South Wales and Australian teammate Ricky Stuart.
In his final season before retiring, Mullins played 26 games for the Roosters (mostly on the wing) and scored 17 tries to take his tally to 122 tries from 209 premiership matches in Australia.
Despite playing on the wing for New South Wales in their 2-1 Origin series win over Queensland in 1994 (and despite the good form of incumbent Blues fullback Tim Brasher), Mullins made his test début at fullback for Australia in a one-off Test against France on 6 July 1994 at the Parramatta Stadium in Sydney, scoring a try in the Kangaroos record 58–0 win.
Towards the end of the year Mullins, along with fellow Canberra test players Laurie Daley, Ricky Stuart, Steve Walters and Bradley Clyde took the ARL to court in order to be ruled eligible for selection at the 1995 Rugby League World Cup to be played in the United Kingdom.