After starting his career in non-league football he moved to Barnet, Gillingham, Nottingham Forest, and later Watford, whilst also spending a loan spell at Leeds United.

King started his career at Dulwich Hamlet before becoming an apprentice at Barnet Football Club.He made 60 first team appearances for the London club before making the move to Gillingham in 2000.At Gillingham he quickly established himself as the Club's main striker after being given chances through injuries to key players.

One of his career highlights came with the Gills in 2002 when he scored against Chelsea at Stamford Bridge in the FA Cup.

In 2003 he moved to Nottingham Forest and scored 15 goals in 57 games, but was loaned out to Leeds in March of 2005 for the remainder of the 04/05 Season.

King went on loan to Watford before the start of the 2005/06 Season, Watford also had an option to buy in January.He impressed the Hornets scoring 12 goals in 21 games and made his move permanent with a £500,000 deal in the transfer window.He went on to finish the Championship's top scorer that season with 21 goals and was also named 'Player of the Season'.