He was picked up by Manchester United at the age of 12, where Walker was part of the second generation of Busby Babes who were brought through the club after the Munich air disaster. He was on the verge of being selected for the England schoolboys under-15 team, which would have made Walker the first black player to represent England at any level, however he was not eligible as he had already signed professional terms with his club. He became a full-time professional in November 1961. Walker made his Manchester United debut against Nottingham Forest on 20 May 1963, the last game of the 1962–63 Football League First Division season, when Matt Busby elected to rest several first team players ahead of the 1963 FA Cup Final; Walker thus become United's first ever black player.