Brett played early football in the backyard with his brother, Craig, and then for the Lakes United Seagulls and Valentine-Eleebana Red Devils.

After limited opportunities in first-grade at his favoured half-back position with the Knights (because of Andrew Johns), Kimmorley opted to sign for newly formed Super League club the Hunter Mariners who were also based out of Newcastle midway through the 1997 split season.

Melbourne claimed their maiden premiership, with Kimmorley leading a spirited Storm comeback and setting up the match-winning try with a chip-kick to earn the Clive Churchill medal for best on-ground.

Having won the 1999 Premiership, the Melbourne Storm travelled to England to contest the 2000 World Club Challenge against Super League Champions St Helens R.F.C., with Kimmorley playing at halfback in the victory.

In 2000, Kimmorley made his State of Origin debut for New South Wales, helping the Blues to a clean sweep, and keeping Johns on the bench for Games II and III.

[citation needed] With the new season in 2003 looking promising, Kimmorley was awarded the captaincy of the team over David Peachey and was rewarded with a new five-year deal.

He was also plagued with several injuries which caused him to miss the annual State of Origin series fixtures, although he was selected in the Australian team to go and compete in the end of season 2004 Rugby League Tri-Nations tournament.

[7] The start of 2005 looked extremely promising for the club, with Cronulla and Kimmorley winning the first 6 games in a row to be on top of the NRL Ladder.

However, after Kimmorley's selection for New South Wales Rugby League team in the first State of Origin match, in which he threw the fateful intercept ball to hand Queensland a victory, his confidence was seemingly shattered and his club form suffered badly.

Kimmorley was selected as halfback for the 2007 New South Wales State of Origin team for games 2 and 3, after Jarrod Mullen was dropped due to injury.