Born in Hastings, East Sussex, Cook made his first-team debut for Brighton & Hove Albion during the League Cup third round victory over Manchester City on 24 September 2008. He made his second substitute appearance for Brighton during a 2–1 FA Cup first round replay defeat against Hartlepool United on 18 November 2008. On 23 December 2008, Cook joined Conference South team Havant & Waterlooville on loan for six-weeks. When this loan expired, he once again found himself involved in the Brighton first team, making his third substitute appearance for the club during the Football League Trophy Southern Final penalty shoot-out defeat to Luton Town. Cook made his first league appearance for Brighton during a 4–0 home defeat to Crewe Alexandra on 28 February 2009. He, along with fellow youth players Steve Brinkhurst and Josh Pelling, signed professional contracts with Brighton on 7 May 2009. On 20 November 2009, Cook joined Conference South team Eastleigh on loan to gain first-team match experience. He was sent off on his debut in a 1–1 draw with Lewes in the third qualifying round of the FA Trophy on 21 November 2009. On 16 September 2010, Cook once again went on loan, this time to Conference Premier side Eastbourne Borough where he spent a month. After making seven appearances and scoring once, Eastbourne were keen to extend the loan deal for the player, but were however unsuccessful as he later joined Conference Premier rivals Mansfield Town on a loan deal lasting until January 2011. Brighton manager Gus Poyet later revealed that he felt Cook would add another ingredient to his footballing education by spending time away from Sussex and that it would force him to "grow up". Cook made his first appearance for Brighton in over two-and-a-half years during the League Cup game against Liverpoolin September 2011.
Bournemouth
On 27 October 2011, Cook joined League One side Bournemouth on loan. On 3 January 2012, Cook re-joined Bournemouth permanently for £150,000 signing a three-and-a-half-year deal. Cook was ever-present in the 2014–15 season as Bournemouth won the Championship and were promoted to the Premier League. On 4 December 2016, he scored to equalise and then created the winning goal as Bournemouth completed one of the comebacks of the season to beat Liverpool for the first time in their history 4–3 in a sensational Premier League game. He made his 300th appearance for the Cherries in February 2019. On 18 January 2020, Cook received a red card against Norwich City, for a handball.
