Early career King started his career at
Dulwich Hamlet, before becoming a trainee at
Barnet, where he would go on to make 60
first team appearances In August 2001 he signed a new four-year contract with the Kent side. In May 2002 King was jailed for 18 months for driving a stolen car, but returned to the Gillingham team in October 2002 after he was released on appeal. He scored 15 goals in 57 games. He struggled to win over the fans at Nottingham Forest, before netting an injury-time winning goal in an emotional match against
West Ham United, the first after the death of
Brian Clough. King was
loaned to
Leeds United in March 2005 for the remainder of the 2004–05 season. Whilst at Leeds, manager
Kevin Blackwell mainly played King out of position on the wing, and as a result King failed to net a single goal for Leeds, including missing a
penalty kick against
Leicester City. In total he played nine games for Leeds and returned to Nottingham Forest in the summer of 2005 after his loan spell ended.
Watford In the summer of 2005 King was loaned to
Watford for six months, with an option to buy in the January transfer window. He impressed at the
Hertfordshire club, scoring 12 goals in 21 games as Watford featured in the upper reaches of the table. King scored his first goal for the Hornets in a 3–3 draw away at
Plymouth Argyle on 9 August, and followed this up with two goals at
Cardiff City in his next match. and King went on to finish top scorer in the
2005–06 Championship with 21 goals. Watford finished third, with King scoring in the 3–0 semi-final first leg win over
Crystal Palace. He played in the second leg, and in the final against Leeds United, which Watford won 3–0 to win promotion to the
FA Premier League. King was named the club's
Player of the Season. King scored his debut Premier League goal against West Ham United in the second game of the new season. In the following game, away at
Arsenal, he sustained a knee injury which kept him out of the next three games. During his return to fitness he suffered a relapse, and in an exploratory operation it was discovered that some bone had flaked away from his
femur. The injury kept him out of action for six months, and he did not return until 14 April 2007, when he came on as a
substitute in the 4–1
FA Cup semi–final loss to
Manchester United at
Villa Park. He scored in the final two games of the season, away at
Reading and at home to
Newcastle United to bring his season tally to four. Following King's conviction in October 2009 Fulham manager
Roy Hodgson said "I think we got lucky in that we didn't sign him. There was a problem with the medical and it also became a problem when we found out he had a jail sentence.
Mohamed Al Fayed was not at all keen on that so the transfer was allowed to collapse and he went to Wigan instead." He signed for
Wigan Athletic on a three-and-a-half-year contract for an undisclosed fee on 25 January 2008. Watford chairman
Graham Simpson said that King had left for a fee of £3million rising to £4million, dependent on appearances and Wigan keeping their Premier League status, and it was reported that King doubled his wages to £40,000 per week. in 2009 In August 2008, Premier League newcomers
Hull City bid an undisclosed amount for King, which Wigan accepted. City agreed personal terms with King, but the move fell through as he was unable to negotiate the terms of his exit with Wigan. Hull instead took King on a season-long loan. He scored his first goal for City, a
penalty, on 13 September against
Newcastle United, adding a second goal in the second half to win the match 2–1. King's loan at Hull was cut short after he reacted in an "unacceptable manner" when told he was not in the starting line-up for a match against Arsenal, and he was sent home rather than taking his place among the substitutes. He made his debut against
Chelsea on 28 January. He scored his first goal for Middlesbrough on 14 March against
Portsmouth. King grabbed a goal against his old club Hull on 11 April in a win for the Teessiders. However, the season ended with Middlesbrough's relegation from the Premier League. King returned to Wigan in the summer of 2009 season after his loan spell ended. He mainly found himself on the bench for
Roberto Martínez's side. He was sacked in October 2009, following his conviction and sentencing for sexual assault. King scored his first goal for the Sky Blues from the penalty spot to win the game against Middlesbrough on 4 December, and finished the season as the club's top scorer with 13 goals. He was also chosen Player of the Year.
Birmingham City With his Coventry contract about to expire, King was reported to have verbally agreed a new deal with the club. However, on 10 June 2011, he agreed a three-year contract with
Birmingham City, citing the opportunity to play in the
Europa League as a major factor in his decision. The free transfer, under the
Bosman ruling, was formally completed on 1 July. Coventry's chairman and manager felt "betrayed" by the player's actions. He suffered knee ligament damage in pre-season training which delayed his Birmingham debut until 11 September, when he played the last half-hour of a 3–0 home win over
Millwall. King scored his first goal in European competition on his first start for Birmingham, in the 3–1 home defeat to
Braga in the
Europa League group stage, and his first League goal for the club on 16 October, a penalty to open the scoring in a 2–0 win against
Leicester City. King finished the season with 18 goals and 12 assists as Birmingham reached the
play-off semi-final. King scored the first senior
hat-trick of his career to secure a draw after Birmingham were 3–0 down after 19 minutes away to Millwall in October 2012. This was the start of a scoring run of nine goals in as many games, finishing with Birmingham's 3–2 defeat of third-placed Middlesbrough in December, in which King was fouled for and converted a penalty just before half-time to level the scores, and ran onto a through ball by
Peter Løvenkrands to score the late winner with what the
Birmingham Mail described as a "clinical finish". Although put up for sale in January, a bid from
Shanghai Shenhua was rejected in February. However, in March, his season was brought to a premature close due to a long-term knee injury deteriorated, requiring an operation. On 28 August 2013, it was announced that Marlon King would be leaving Birmingham, after his contract was terminated by mutual consent in order to reduce the club's wage bill.
Sheffield United Having initially been rejected by the club, in September 2013 King signed deal with
Sheffield United to remain with the
League One club for the remainder of the season. Having made his United debut in a 1–0 home loss to
Preston North End, King quickly fell out of favour following the arrival of new manager
Nigel Clough however, and following a spell on the sidelines due to a knee injury he was released in December 2013. ==International career==