During his time at Nottingham Forest, Cohen was known for his energetic and hard-working style of play and the consistency of his performances which won him multiple player of the season awards.

Cohen was a product of the West Ham United youth system and he was scouted whilst playing for an amateur football club at the age of six.

[10] Following Yeovil's exit from the FA Cup against Rushden and Diamonds, manager Russell Slade exempted Cohen from criticism, praising his performance.

[7] In a combined £1.2 million deal with Yeovil teammate Arron Davies, Cohen moved to League One side Nottingham Forest on 6 July 2007, signing a four-year contract.

[16] He then hit a rich vein of form by helping Forest achieve an eight-game unbeaten run to put them into second spot in the league at Christmas 2007, including an influential display in the 3–0 win at Cheltenham Town.

In the 2008–09 season, Cohen continued to impress with consistent displays, and was picked out for praise by visiting Charlton manager Alan Pardew after a 0–0 draw, as being the best player on the pitch.

Manager Billy Davies said of Cohen, "He's a player with a wonderful attitude, can play in several positions, has tremendous energy and has got outstanding ability".

[20] In the first leg of the 2009–10 Championship play-off semi-final against Blackpool on 8 May 2010, Cohen put Forest ahead with a curling outside-foot volley at Bloomfield Road.

[29] On the back of his performances and longevity with Nottingham Forest, the 26-year-old midfielder was made the club captain on 31 July 2013, succeeding Danny Collins.

[30] He made an immediate impact as captain, as he was an important part in Forest's first goal of the 2013–14 season, making a great run to the byline and pulling it back for Henri Lansbury to score against Huddersfield.

Cohen limped off late in the game and it was subsequently confirmed that he had suffered cruciate ligament damage, which ruled him out for the rest of the season.

Having missed fifteen months of football, Cohen eventually made his return to the first team to "huge cheers" on 2 January 2016, when he replaced Jack Hobbs in the 86th minute of a 1–1 draw at Charlton Athletic.

[39] Cohen started Forest's next game at left-back; a home win in the FA Cup over Queens Park Rangers on 9 January.

[41] Cohen began the 2016–17 season well under the new management of Philippe Montanier, but missed Forest's 2–0 win over Ipswich Town at Portman Road on 19 November with an injury.

It was subsequently confirmed that Cohen had sustained a groin injury in training that required surgery, and would be expected to keep him out of action for three months.

[42] Cohen returned from injury to play a pivotal role in Forest retaining their Championship status in the final game of the season.

[49] On 10 November 2022, Cohen departed Luton Town to take on a first-team coach role at Premier League club Southampton, following Nathan Jones in the move.

[51] In November 2023, Cohen joined League One side Lincoln City to assist Interim Head Coach Tom Shaw.