Turnbull was born to John Turnbull and Kaye Holdsworth on 23 September 1971, in Mount Lawley, Western Australia. His father played 209 games for Swan Districts in the WAFL, including the 1961, 1962 and 1963 premierships. His mother was the daughter of Ted Holdsworth, who played 143 games for Swan Districts, kicking 532 goals. Turnbull was educated at Christ Church Grammar School in Claremont, Western Australia, and played in the school's First XVIII in 1987. He also played junior football for the Karrinyup Saints Junior Football Club.
Turnbull played for the University Football Club in the Western Australian Amateur Football League (WAAFL) in 1989 before making his debut for Claremont in the WAFL midway through the season. He played in Claremont's 1989 premiership before being selected by the West Coast Eagles with a compensatory pre-draft selection, along with Peter Mann. In the 1990 season, he did not manage to break into West Coast's senior side, or hold his place in Claremont's senior side, but ended up playing in Claremont's 1990 reserves premiership. Turnbull switched to East Perth in the WAFL for the 1991 season after a difference of opinion between Claremont coach Gerard Neesham and West Coast coach Michael Malthouse on how he was being used at WAFL level. Turnbull made his debut for West Coast in round seven of the 1991 season, against Hawthorn at Princes Park, as the youngest member of the team, gathering nine disposals and effecting four hit-outs partnered with Dean Irving. He played four further games in the 1991 season, being kept out of the side by older, more experienced key-position players such as Michael Brennan, Ashley McIntosh, Glen Jakovich, Dean Irving and Stevan Jackson. Turnbull won the F. D. Book Medal in 1991 as East Perth's best and fairest.
Turnbull managed six games in 1992, including five of the Eagles' first seven, but was again kept out of the side by first-choice ruckman Paul Harding. He did not play for the Eagles in 1993, mainly due to the form of Irving and Harding, but managed to win a second F. D. Book Medal. He was a favourite for the 1993 Sandover Medal, but lost to Robbie West, finishing seventh overall. He broke into the Eagles' side in the 1994 season, playing in all 23 of the Eagles' games, including the 1994 Grand Final, which the Eagles won by 80 points. He received his first votes for the Brownlow Medal in round 18, 1994, gaining one vote for 23-hit-out game against Richmond. Turnbull once again played in all of the Eagles' games in the 1995 season as their first-choice ruckman. He played the first game of the 1996 season, but was dropped for Jason Ball. He returned to the side in round 11 after kicking seven goals in seven games with East Perth, and played all but one of the Eagles' remaining games, filling in for the injured Glen Jakovich at centre half-back. He was named as the ruckman in the Eagles' Team of the Decade, announced in 1996 to coincide with the AFL's centenary celebrations. He again played in every West Coast Eagles game in 1997, playing behind Michael Gardiner as the Eagles' back-up ruckman.
Turnbull played the first seven games of the 1998 season, again as a back-up ruckman to Gardiner, but was dropped after a round 7 game against Adelaide where he only managed one disposal and one hit-out. He was recalled for the round 12 game against Hawthorn, but re-dropped after again managing only one disposal and one hit-out. He spent the rest of the 1998 season with East Perth in the WAFL. Turnbull did not play for the Eagles in the 1999 season, with Gardiner and Jason Ball the first-choice ruck combination. He played 17 games for East Perth in the WAFL, and was considered one of the favourites for the Sandover Medal, eventually losing to Gus Seebeck by one vote. He also won a third F. D. Book Medal as East Perth's best and fairest. Turnbull re-established himself in the Eagles' team for the 2000 season, playing all 22 games. He played as the first-choice ruckman in the second half of the season after an injury to Gardiner.
Turnbull was unable to regain a place in the Eagles' side in the 2001 season playing only three games, behind Gardiner, Dean Cox and David Sierakowski, before being de-listed at the end of the season. He played 17 games for East Perth in 2001, including in the 2001 premiership team, winning the Simpson Medal as the best on ground in the Grand Final. He also won the Sandover Medal for the best player in the WAFL. Turnbull played 19 games for East Perth in 2002, again winning the Simpson Medal in East Perth's Grand Final victory. He continued to play for the Royals in 2003 and 2004, but was impacted by an eye injury sustained in the 2003 finals series. He retired from football at the end of the 2004 season.
Turnbull is currently a commentator for ABC Sport, commentating on WAFL coverage. In 2010, he was inducted into the East Perth Football Club's Hall of Fame.