He began his career at Preston North End and later played for Southend United, Charlton Athletic and Swindon Town before signing for Brentford in 2013.
[4] He joined Leyton Orient of the Third Division on 29 August 2003 on a one-month loan and made the first professional appearance of his career in a 4–1 home defeat to Cheltenham Town the following day,[5] when he replaced Billy Jones at half-time.
[10] He joined League Two club Southend United on loan on 16 March 2005 and made his debut three days later, starting in a 3–0 victory over Cheltenham Town.
[9] He spent the entire 2005–06 season on loan at Motherwell of the Scottish Premier League,[14] making 28 appearances in all competitions, scoring two goals, accumulating 12 yellow cards and one red.
[15] McCormack made his first Preston appearance in nearly two years when he replaced Kelvin Wilson after 41 minutes of a 1–1 Championship home draw with Queens Park Rangers on 19 August 2006.
[17] Looking back in August 2012 on his time with Preston, McCormack said "I had a great few years there and I learnt a hell of a lot – enough to help me go on and continue my career in the professional game in England.
[19][20] After an impressive first month of his spell, assistant manager Paul Brush confirmed on 11 December that McCormack would transfer to the club permanently and he subsequently signed on a two-and-a-half-year contract.
[29] McCormack played the full 90 minutes in a 1–1 draw with Chelsea at Stamford Bridge in the FA Cup third round on 3 January 2009,[30] but missed the replay, which Southend lost 4–1.
[40] He enjoyed a successful first season with the club, making 51 appearances, scoring two goals, winning the League Two championship and the Swindon Town Player of the Year award.
[43] Under the management of Paolo Di Canio, McCormack had a good 2012–13 season back in League One, being named captain,[18] making 47 appearances and helping Swindon to a sixth-place finish which saw the club qualify for the play-offs,[44] where they were defeated on penalties by Brentford in the semi-finals.
[49] He began the 2013–14 season in his natural midfield position, before being moved to right-back by manager Uwe Rösler in October, to replace out-of-form Shay Logan.
[51][52] A near ever-present under Uwe Rösler and his successor Mark Warburton, McCormack celebrated promotion to the Championship after a 1–0 victory over former club Preston North End on 18 April 2014.
[57] His status as an ever-present in league matches came to an end after he suffered an ankle ligament injury during a 3–1 defeat to Bolton Wanderers on 25 October 2014.
[58] He returned to light training in January 2015,[59] but had lost the right-back position to Moses Odubajo and made just four further appearances before the end of the 2014–15 season.
[64][65] McCormack finally returned to the team for a West London derby versus Queens Park Rangers on 12 March 2016 and he played the full 90 minutes of the 3–0 defeat.
[63] McCormack made his first appearance of the 2016–17 season in a 1–0 defeat to League Two team Exeter City in the EFL Cup first round on 9 August 2016, in which he was substituted in the 12th minute after suffering a foot injury.
[72] He scored with a 25-yard volley on the opening day of the 2017–18 season in an 8–2 win at home to Yeovil Town,[73] which was named the League Two Goal of the Month for August 2017.
[74] He was a regular in the team during the first six weeks of the season, starting in every league match until suffering a groin injury in a 2–1 away victory over Wycombe Wanderers.
[83] He made his return on the final day of the season, playing 82 minutes to help Luton to a 3–1 home victory over Oxford United and the League One title.
[1] Injury to Darren Potter saw McCormack called up to the Republic of Ireland under-19 team for three 2003 European U19 Championship second qualifying round matches in May 2003.