A product of the Manchester United youth academy, Cathcart spent time on loan at Belgian club Royal Antwerp, and English Football League Championship sides Plymouth Argyle and Watford, before joining newly promoted Blackpool in 2010.

Cathcart returned to Watford in July 2014 on a permanent deal where he became one of their longest serving players, achieving two promotions and an FA Cup Final.

[6][7] 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.

[13] 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.

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.

[15] He then came on as a 66th-minute substitute for 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.

[18] 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.

[19] 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.

[20] 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.

[27] 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.