Cullen signed for the Clarets in the summer of 2022 penning a three-year deal. Born in Essex, the midfielder played for Rayleigh Boys before joining West Ham’s academy at the age of nine, remaining a Hammer for 15 years. Representing England U16 in 2011, Cullen made his first-team debut at club level as a substitute four years later. It came in a UEFA Europa League qualifying round match against Andorran side Lusitanos, with Cullen starting the return leg a week later. Soon after came his Premier League debut, again as a substitute, away at Anfield with Liverpool the opponents. Loaned out to Bradford City in February 2016, Cullen suffered defeat in the Play-Off final, but returned on a season-long loan for the following campaign. Making 63 appearances for the Bantams across that 18-month spell, he scored his first senior goal against MK Dons. In the same time, he started to make moves internationally, switching allegiance to the Republic of Ireland, qualifying through heritage. The midfielder played for the Boys in Green at U19 & U21 levels, and is now a full international, making his debut at first-grade in September 2019. Returning from Bradford, Cullen went back out on loan into the Football League with Bolton Wanderers. In the North West for half a season, he made 12 appearances for the Trotters before ending the season back with the Hammers in the Premier League. 2018/19 then saw a highly successful loan to Charlton Athletic, winning promotion to the Championship via the Play-Offs at the first time of asking. Cullen returned to the Valley the following season, tallying 66 appearances across the two seasons. After 15 years with West Ham, Cullen eventually joined Anderlecht in October 2020 where he linked up with current Clarets boss Vincent Kompany. Spending two seasons in Belgium, the midfielder captained the side and amounted a further 77 games before moving to Turf Moor in the summer of 2022. Cullen enjoyed a successful first season with the Clarets, playing 50 games in total and helping the side seal the Sky Bet Championship title. The midfielder also picked up the Player of the Year and Players Player of the Year awards at the end of season ceremony before the Champions open-top bus parade. Cullen then committed his future to the Clarets in August of 2023 after signing a new three-year deal at Turf Moor. The Ireland international played 25 games in the Premier League, scoring twice including a goal at Stamford Bridge in a 2-2 draw at Stamford Bridge last season. The 2024/25 season saw Cullen feature in 44 Championship matches where he contributed two goals and one assist from the midfield, playing a key role in Burnley's dominant promotion campaign.