Shearer was born in St George, Queensland.
At the age of 18, Shearer caught the attention of many people during his time at the Queensland club Sarina Crocodiles in the Winfield State League with some impressive performances. Shearer was a member of Mackay's second successful Foley Shield team in 1984, as well as representing Queensland in a match against New Zealand.
The Manly-Warringah Sea Eagles coach and rugby league Immortal, Bob Fulton, was quick to sign the 19-year-old before anyone else prior to the 1985 NSWRL season. Shearer made his début for Manly in round 1 of the season against Penrith, before going on to make his State of Origin début for Queensland in the same year. Playing mostly on the wing or in the centres, Shearer played 22 games in his début season for Manly, scoring 8 tries. It would prove to be the most number of club games he would play in a season in his 14-year career. He scored his first try for Manly at their home ground, Brookvale Oval, in their 24-4 win over the reigning premiers Canterbury-Bankstown in round 3.
He made his test début on the wing for Australia in 1986 against New Zealand at Carlaw Park in Auckland, scoring a try on debut. Though an injury suffered in the game saw him unavailable for the final two tests against New Zealand at the Sydney Cricket Ground and Lang Park in Brisbane. His good form for Manly, Queensland and Australia saw him selected on the 1986 Kangaroo Tour where he played 13 games on tour, including two tests against Great Britain and two against France, scoring 12 tries. He scored a record four tries in the second test against France in the final match of the tour, in a then record 52–0 win.
Although played at either fullback or in the centres for Manly by Fulton, Shearer was picked on the wing for both Queensland and Australia in 1987, helping the Maroon's to regain the State of Origin shield from New South Wales, and playing in Australia's shock 6–13 loss to New Zealand at Lang Park. He was Manly's starting fullback in their 18-8 Winfield Cup grand final win over the Canberra Raiders in the last ever grand final played at the Sydney Cricket Ground in 1987. Manly had finished the season as minor premiers with a 20-5-1 record (which included a 12-game winning streak), with Shearer playing 20 games (14 at fullback and 6 in the centres) and scoring a career high 13 tries to be the clubs leading try scorer for the season. Following the grand final victory he travelled with Manly to England for the 1987 World Club Challenge against the English champions, Wigan, at Central Park.
In the following off-season he also played 14 games in the championship-winning Widnes team in England during the 1987–88 RFL championship. Shearer played for Manly until 1989
Shearer joined the Brisbane Broncos. He was selected to play for Queensland in all three games of the 1990 State of Origin series at centre. Shearer was the 1990 Brisbane Broncos season's goal kicker. He was selected to go on the 1990 Kangaroo tour of Great Britain and France. Shearer was selected to play for Queensland from the interchange bench in the 1991 State of Origin series' 2nd and 3rd games. He was then selected to play for Australia in the first match of the 1991 Trans-Tasman Test series on the wing.
Shearer then joined the Gold Coast Seagulls. He was selected as Queensland's fullback for all three games of the 1992 State of Origin series. He played for Queensland at fullback in games 2 and 3 of the 1993 State of Origin series. Shearer was Australia's fullback and goal kicker in all three matches of the 1993 Trans-Tasman Test series. He played his last match for Australia in 1993. Shearer then joined the South Queensland Crushers. He was selected to play for Queensland at five-eighth in the 1995 State of Origin series. He played his last game for Queensland in 1996. Shearer later joined the Sydney Roosters. The 1998 North Queensland Cowboys season was Shearer's last.
In 2000 he was awarded the Australian Sports Medal for his "outstanding achievement in rugby league". In August, 2008, Shearer was named at wing in the Indigenous Team of the Century, joining other legends such as Eric Simms, Steve Renouf, Lionel Morgan, Laurie Daley, Greg Inglis, Jonathan Thurston, Arthur Beetson, Cliff Lyons and John Ferguson in the team.
Shearer was critically injured in a car crash at Peregian Springs on 21 February 2009. The crash came one year after the death from cancer of his wife Delyse.
In 2010 Shearer faced bankruptcy when ordered to repay approximately $1.5m to former State of Origin teammate Adrian Lam for an outstanding loan which dated back to 2005.