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.

[5] He remained relatively unscathed by The Troubles, for his Protestant parents kept a firm watch on their children to ensure that they did not stray far from home and that none of them became involved with Ulster loyalism.

[14] His injury problems started as early as 1980, when he was 15 years old and saw Glentoran's physio Bobby McGregor to resolve a groin strain; Whiteside said that the highly physical massage to which he was subjected damaged his pelvis and robbed him of his natural pace.

[16] Six weeks into his apprenticeship, in July 1981, and an innocuous looking challenge by a Preston North End player in an 'A' team game saw Whiteside requiring an operation on his right knee.

[19] He had to adapt his playing style to compensate for his damaged knee and pelvis,[20] and developed well under youth team coach Eric Harrison.

[21] Whiteside became United's youngest first-team player since Duncan Edwards in 1953, when he made his debut as a substitute against Brighton & Hove Albion in a 1–0 league win at the Goldstone Ground on 24 April 1982, two weeks before his 17th birthday.

[24] He turned professional at Old Trafford on his 17th birthday, signing a three-year contract,[25] and he shared a room with Steve Coppell, who was heading into retirement due to a knee injury of his own.

[28] His performances at the World Cup convinced manager Ron Atkinson to start Whiteside alongside Frank Stapleton at the front of United's attack for the 1982–83 season.

[29] The pair were similar players in that they held the ball up and made frequent runs into the box, allowing midfielder Bryan Robson to join in the attack.

[37] He continued to impress, despite on-loan striker Garth Crooks being tipped to take his first-team place and sign a permanent contract (which never happened).

[40] He lost his starting place to Mark Hughes for the semi-final clash with Juventus at the Stadio Olimpico di Torino, which ended in a 2–1 defeat.

[46][47] The goal required vision, timing, and precise shooting;[48] and The Guardian praised his "cool footballing brain" and "clever concealment of the shot".

[61] Whiteside's goal in a 2–1 win over Derby County in a league match at the Baseball Ground on 10 February 1988 would prove to be his last for Manchester United.

[62] In the summer of 1988, Ferguson brought striker Mark Hughes back to Old Trafford after two years away, further limiting Whiteside's opportunities for the first team.

His continued fitness problems, and the growing competition for places in the first team, meant that Whiteside played just six league games in the 1988–89 season.

[64] Ferguson intended to build his own team having signed midfield duo Neil Webb and Mike Phelan, and also in the hunt for other players including Paul Ince, Gary Pallister and Danny Wallace.

[65] He had played a total of 274 games in all competitions for United, scoring 67 goals, and collected two winner's medals in the FA Cup, despite only being 24 years old when he left.

[69] Manager Colin Harvey was planning to boost the "Toffees" chances of more silverware, and also in 1989 signed Mike Newell, Martin Keown, Stefan Rehn and Raymond Atteveld.

[78] Five days later he helped his side to a 1–1 draw with Honduras, which was a disappointment, and many believed had doomed Northern Ireland's chances of advancing in the competition.

[80] However a mistake from goalkeeper Luis Arconada gifted Gerry Armstrong the only goal of the game, and despite having Mal Donaghy sent off on 60 minutes, Northern Ireland went on to record a 1–0 win.

[82] A win against France would take them into the semi-finals, however a French team inspired by Michel Platini won 4–1 and therefore eliminated Northern Ireland from the competition.

[86] At the 1986 FIFA World Cup finals in Mexico, he scored with a deflected free-kick in the 1–1 draw against Algeria at the Estadio Tres de Marzo.

[93] Upon retirement, Whiteside studied to become a podiatrist, graduating with a degree from the University of Salford, and served Northwich Victoria as their assistant manager/physio from October 1991 until March 1992.

[99] With the help of writer Rob Bagchi, he released his autobiography entitled Determined in August 2007, published by Headline, and with a foreword by actor James Nesbitt.

[100] When Saturday Comes magazine reviewer Joyce Woolridge wrote that "Determined is an entertaining, well written account of one of the less ordinary 1980s footballers, with the added twist of how Whiteside was able to rebuild his life, if not his knee.