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. 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, he scored his first goal for his new club two games later in a 1–1 draw with Crawley Town at Bramall Lane. 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. In 2004 King played an important part in Jamaica's unsuccessful attempt to qualify for the 2006 World Cup, scoring six goals in eight games. He played for Jamaica in the friendly against Ghana on 29 May 2006 which they lost 4–1. He was named in the squad to face England in a friendly on 3 June 2006, but was sent home for an alleged breach of discipline. Jamaica lost the match 6–0. As a result of the incident, the Jamaican Football Federation banned King from international football until May 2008. When Captain Horace Burrell returned as the JFF president he lifted the ban. King played in Jamaica's 1–1 draw with Costa Rica in February 2008, where he provided the cross for Jamaica's goal. King also played for Jamaica in a friendly against Trinidad and Tobago scoring a goal and captaining the team in the absence of Ricardo Gardner due to injury.