Born in Belfast,[2] McCartney was a trainee with Sunderland before signing a professional contract on 28 May 1998 and made his debut against Luton Town in the League Cup in 2000.

[1][3] He was made team captain in 2003–04 after Jason McAteer suffered a long-term injury and in 2004–05 was voted Player of the Season by Sunderland fans during the successful Championship winning campaign.

[5] He showed glimpses of his potential once he had regained his fitness, but his return came far too late in the season for him to have any meaningful impact on the team's dreadful run of results.

McCartney's sale back to his former club Sunderland led to West Ham manager Alan Curbishley resigning after he claimed the player was sold against his wishes.

[citation needed] In order to help him regain match fitness after suffering a knee injury in pre-season and losing his first-team place at Sunderland, he joined Leeds United on 23 September 2010, initially on a one-month loan, a move which saw him link up with Northern Ireland assistant manager Glynn Snodin.

[18] In his second game, Leeds suffered a 6–4 defeat to Preston North End, and McCartney gave away a penalty in the match which was scored by Callum Davidson.

[30] On 19 December, McCartney was quoted as saying although Martin O'Neill had taken over at his club Sunderland, he had had no contact with the manager, and "To be honest, before I agreed to come to West Ham, it was decided that my contract would be left to run out in the summer and sorted out at the end of the season.

[34] On 1 July 2012 McCartney rejoined newly promoted West Ham signing a two-year contract after agreeing to make the loan move permanent.

He made his comeback on 30 March 2013 against West Bromwich Albion but lasted only 15 minutes before a recurrence of the same injury which was expected to keep him from playing for the remainder of the 2012–13[36] season.

[40] He made his debut for the senior team in September 2001 in their World Cup qualifying win against Iceland, scoring his only goal in his professional career until that for West Ham in 2007.

[45] During the 2021–22 NIFL Premiership season, McCartney joined Linfield as a coach, reuniting with former Northern Irish teammate David Healy.