Having been in their youth schemes since the age of 10, Campbell signed for the Manchester United Academy on 1 July 2004. He made an immediate impact, scoring 14 goals in 22 starts for the Under-18s in the 2004–05 season. His performances for the youth team also led to him playing five times for the reserves, for whom he scored once. He signed his first professional contract with Manchester United on 22 March 2006, and he was named as a substitute for Roy Keane's testimonial match at Old Trafford on 9 May 2006, in which he was a 75th minute replacement for Kieran Richardson. Later that summer, he scored his first goal for the club after coming on as a substitute for Wayne Rooney in a friendly against Macclesfield Town.