Martin began his career as a trainee with Wimbledon, where he caught the eye of many Premier League teams. Pursued by a number of other top clubs, including Arsenal, Liverpool and Tottenham Hotspur, he signed for Manchester United on 17 December 2003, following a week-long trial in October 2003, during which time he made one appearance for the Manchester United Under-19s team. The compensation package United paid Wimbledon is estimated to be approximately £1 million, with £200k being paid up front, and the rest in instalments based on both his and Manchester United's performance.