Early career Born in
Belfast, he began training at the
Manchester United School of Excellence in Belfast. He signed trainee terms at Manchester United in July 2005, and turned professional on his 17th birthday in February 2006. Cathcart became a regular in the Manchester United Under-18s team, and was a member of the Under-17s team that played at the 2007 Northern Ireland Milk Cup, scoring the first goal in a 5–0 win over Liechtenstein. During the 2006–07 season, he was made captain of the United youth team that reached the
FA Youth Cup Final, although he missed both legs of the final, the first through selection for the first team and the second through injury.
Manchester United On 19 March 2007, Cathcart received his first call-up to the Manchester United senior squad for the
FA Cup Sixth Round replay with
Middlesbrough. After an injury to
Nemanja Vidić, Cathcart was promoted to the first team squad as cover on 31 March 2007. He was an unused substitute in Manchester United's 7–1 win against
Roma in the
Champions League less than two weeks later, the
FA Cup semi-final victory over
Watford a further four days later, and the victory over
Sheffield United on 17 April. Speculation that a defensive injury crisis would lead to Cathcart making his senior debut had ended, when he suffered a season-ending knee injury during training after the match against Sheffield United. As a result, he also missed the second leg of the FA Youth Cup final; he had already missed the first leg due to his call-up to the senior squad. Despite his injury, Cathcart's performances in the club's youth teams earned him the
Jimmy Murphy Young Player of the Year award for the 2006–07 season.
Loan to Antwerp At the start of the 2007–08 season, Cathcart was sent on a half-season loan to United's Belgian feeder club,
Royal Antwerp, to gain match experience. He made his debut for the Belgians on 22 September 2007, in a match against
OC Charleroi. The next game saw Cathcart score Antwerp's only goal of the game in a 4–1 loss to
Kortrijk. He scored again in a match against
Tubize, to earn a 1–1 draw, and also the Man of the Match award. He made a further nine appearances for "The Great Old", to bring his total to 13, before returning to Manchester United at the end of 2007. Although he was not originally selected for the squad, Cathcart was briefly called up to the Manchester United first team for their tour of South Africa on 24 July 2008, and played the full 90 minutes in the team's 4–0 victory over
Kaizer Chiefs on 26 July, his first appearance for the first team. He then came on as a 66th-minute substitute for compatriot
Jonny Evans in a match against
Portsmouth in Nigeria the following day, before then coming on for
Wes Brown at half-time in a friendly against
Peterborough United on 4 August.
Loans to the Championship On 8 August 2008, in a search for first-team football, Cathcart joined
Championship side
Plymouth Argyle, on loan until the end of 2008. This was later extended to the entire 2008–09 season, as Cathcart had quickly become a regular at Home Park. He scored his first Plymouth goal in their 3–1 defeat at home to
Ipswich Town in October 2008. Cathcart returned to Manchester United at the end of the 2008–09 season and played in two pre-season friendlies for the reserve team before joining up with the first team for the
Audi Cup in
Munich. After
Nemanja Vidić suffered a calf strain in the warm-up before the semi-final against
Boca Juniors, Cathcart was promoted to the starting XI. Manchester United won the match 2–1, with Cathcart giving a good performance, although he did head the ball against his own crossbar midway through the second half. Cathcart scored the opening goal in a 3–1 away win over
Bolton Wanderers in the first game of the
2009–10 Premier Reserve League season on 24 August 2009. The following season, Cathcart was sought after by a number of
Championship sides, and Manchester United received a bid of £1.25 million from a Championship side but United preferred a loan deal.
Plymouth Argyle and
Ipswich Town both showed an interest in the young defender, but he instead signed for
Watford on loan until 4 January 2010, where he joined then fellow Manchester United teammate
Tom Cleverley and Arsenal youngster
Henri Lansbury on loan.
Blackpool On 11 August 2010, Cathcart signed for
Blackpool for an undisclosed fee, after agreeing a three-year contract, with an option for a further year. Three days later, he made his debut as Blackpool marked their Premier League debut with a 4–0 win over
Wigan Athletic at the
DW Stadium. He scored his first goal in the Premier League with a powerful header against his former club, Manchester United, at
Bloomfield Road on 25 January 2011; the 15th-minute goal opened the scoring for the match, but Blackpool went on to lose 3–2.
Watford in 2015 On 24 June 2014, Cathcart agreed terms on a two-year deal with
Watford, effective from 1 July, having been out-of-contract at Blackpool. He officially became a Watford player on 1 July. Cathcart's first Watford appearance came as a substitute for
Gabriel Tamaș against
AFC Bournemouth on 20 September 2014, scoring an equalising goal on his debut. In total, he played 29 league games, scoring three times, as Watford were promoted to the Premier League. On 2 July 2015, Cathcart signed a new four-year deal with the club, valid until 2019. On 28 June 2017, Cathcart once again penned a four-year contract extension with the club. On 2 September 2018, Cathcart scored a header in an 2–1 home win against
Tottenham Hotspur. On 12 June 2019, Cathcart extended his contract with the club for another four years, effective until June 2023. On 11 September 2020, Cathcart scored the only goal in a 1–0 win over
Middlesbrough in the Championship. He would later go on to achieve promotion back to the Premier League with Watford at the end of the same
season. On 30 June 2023, after a total of nine years and over 250 appearances, Watford announced Cathcart's departure from the club following the expiration of his contract.
Kortrijk and retirement On 5 September 2023,
Belgian Pro League club
Kortrijk announced the signing of Cathcart on a one-year deal, with an option to extend his contract for another year. However, on 21 September, he decided to retire from professional football early into the season, as he felt he had lost his motivation and enthusiasm for the game, ending a 17-year long career with 510 total appearances made for club and country. ==International career==