Born in Leeds, West Riding of Yorkshire, Liddell started his league career at Barnsley, making his début as a substitute against Portsmouth in 1992. He helped his team achieve promotion to the Premiership in 1997.
Liddell played a part in a significant moment during Barnsley's Premier League season. On the Reds' visit to Anfield to play Liverpool, Liddell created Barnsley's match-winning goal. After being brought down by David James in the penalty area, Liddell could have won a penalty kick but persisted and set up Ashley Ward, who scored via a deflection off Patrick Berger.
He fell out of favour in subsequent years, making most of his appearances from the substitute's bench.
In October 1998, Liddell moved to Wigan Athletic for £350,000, netting ten league goals in his first season at the club. After helping his team to Associate Members Cup success in 1999, he went on to become Wigan Athletic's Player of the Year for the 1999–2000 season.
In the 2001–2002 campaign, Liddell finished as the club's leading scorer, an instrumental part of the team that would reach the play-offs for the second time in succession. During the course of the season, he scored the club's 1,500th league goal and his first career hat-trick against Brighton, following this up with another in a 4–1 defeat of Cambridge United. He received a call-up from Scotland coach Berti Vogts and attended an end-of-season training camp.
Liddell was Wigan Athletic's longest-serving player when he left to join Sheffield United in 2004. He played just under 250 games for Wigan, finishing as the club's leading scorer with a tally of 70 goals, four more than that of David Lowe, the club's previous all-time leading goal-getter.
In his final season for Wigan Athletic he played 38 games and scored 9 goals. When informed his place in the first team for the following season was uncertain, he transferred to Sheffield United, where he played 39 games and scored 6 goals before moving to Oldham Athletic.
Liddell made his 100th league appearance for Oldham on 20 September 2008 in a 3–3 draw with Hartlepool United. Oldham terminated his contract on 7 April 2009, effectively ending his career with the club after a four-year stint in which he made 144 appearances.
Liddell dropped into non-League football at the age of 36 when he signed for Northern Premier League club Bradford Park Avenue in July 2009. Chief executive Bob Blackburn described it as a "huge signing" for Avenue. On 30 July, however, Liddell signed a one-month contract with Rotherham United after impressing whilst on trial. This was later extended to a year but, after only two appearances. because of injury he called time on his playing career. On 22 March 2011, Liddell became Rotherham's caretaker manager for the remainder of the 2010–11 season, after manager Ronnie Moore left the club, to be assisted by Paul Warne and Andy Dibble. He immediately took on Nicky Eaden as a new coach for the club.
He is the son of the late Gary Liddell, a former Leeds United and Grimsby Town striker.