Mullins began his career with First Division side Crystal Palace in 1996, and signed his first professional contract with the club in 1998.

In October 2003, he switched to First Division side West Ham United, initially on a short-term loan, which quickly turned into a permanent transfer.

He remained at the club for five and a half years, making 213 appearances, before joining Premier League side Portsmouth in January 2009.

He made 129 appearances for Portsmouth, but the club's financial difficulties meant that he joined Championship side Reading on loan in March 2012.

After two years, during which he also spent time on loan at Notts County, Birmingham confirmed he would be released when his contract expired at the end of the 2013–14 season.

[1] He joined Crystal Palace as a trainee in August 1996,[2] and made his first-team debut in July 1998, at the age of 19, in the 1998 UEFA Intertoto Cup matches against Turkish club Samsunspor.

[3][4] Head coach Terry Venables gave him his domestic debut in a 3–3 draw against Bolton Wanderers in August 1998, and he scored his first senior goal in a 3–1 away defeat to Birmingham City a week later.

"[2] Financial difficulties meant that Crystal Palace were unable to make new signings for the 1999–2000 season and relied on youngsters such as Mullins and Clinton Morrison and veterans such as Andy Linighan as they successfully battled against relegation from the First Division.

[5] Mullins was the first signing for West Ham's newly appointed manager, Alan Pardew, and initially joined on a short-term loan to enable him to play against Nottingham Forest on the same day.

[22][23] West Ham exceeded expectations in the 2005–06 season, finishing in a mid-table position and reaching the FA Cup final, which was won by Liverpool 3–1 on penalties.

[24] Mullins, having made 42 appearances in that season, was suspended for the final along with Liverpool's Luis García, after the pair had been sent off in a league fixture a few weeks earlier and an appeal against his dismissal was rejected by the Football Association.

"[26] At the beginning of the 2006–07 season, West Ham completed the signing of Argentine international Javier Mascherano who, like Mullins, was a defensive midfielder.

[32] He started the 2010 FA Cup final in an unfamiliar left-back role as Portsmouth were narrowly beaten 1–0 by Chelsea.

[38] He made his debut in the League Cup first round against Barnet,[39] and kept his place for the opening match of the 2012–13 season, a 1–1 draw at home to Charlton Athletic.

[43] With one match of County's season remaining, Mullins was recalled by Birmingham to cover for the injured Tom Thorpe.

[44] Playing at centre back, he made a crucial clearance in the final game, away at Bolton Wanderers, that contributed to Birmingham City retaining their Championship status via Paul Caddis' stoppage-time equaliser.

[53] Mullins took charge against Crystal Palace while newly appointed manager, Nigel Pearson watched from the stands.

This result, coupled with Aston Villa's win later the same day, dropped Watford into 18th place in the league and a relegation position.