Whiteside began his career at Manchester United, signing professional forms in 1982 at the age of 17 and quickly becoming a key member of the side. He scored 68 goals in 278 league and cup appearances for the club over the next seven years, picking up two FA Cup winner's medals in 1983 and 1985, as well as playing in the 1982 FA Youth Cup final, the 1983 League Cup final, and the FA Charity Shield in 1983.