Martyn began his youth career with Kingsmead Boys Club, before then moving on to Valspar Boys Club. Campbell then made the move to Tarff Rovers in the South of Scotland League before signing for Junior side Kello Rovers. It was at Kello that Campbell first caught the eye of Irvine Meadow manager Robert Riley, who then signed Campbell and then brought him to Ayr United with him in 2005 after Riley was appointed Robert Connor's new assistant manager at the club.