Early years and captaincy

Cooper came to the attention of Leeds United after impressing Sporting Director Nicola Salerno in a 2–2 preseason draw at the b2net Stadium in the summer of 2014. After weeks of negotiations, Leeds agreed a deal for £600,000 plus add-ons relating to appearances, a future sale and promotion; with Cooper joining the club on 13 August.^[19] (After winning promotion to the Premier League in 2020, the Spireites received an additional £150,000 from the sale.)^[20]

He made his debut three days later at home to Middlesbrough in a 1–0 win, before being made club captain by Neil Redfearn midway through his first season. He would struggle for consistency under a succession of managers, losing his starting place to Sol Bamba and later to Pontus Jansson, but would still go on to make over 100 appearances during his first four seasons with the club. During a period of 30 games between 2017 and 2018, Cooper was sent off three times and received another six game ban for a stamp on Reading defender Reece Oxford.^[21] His propensity to be a liability in games earned him the disparaging nickname 'League One Liam' among Leeds fans at the time, but Cooper remained a highly respected captain amongst his team mates.^[22] He kept hold of the captaincy during Thomas Christiansen and Paul Heckingbottom's time as manager in the 2017–18 campaign, who would be his 7th and 8th manager respectively at the club in less than four years.

Bielsa and promotion

Cooper retained the captaincy after the shock appointment of the world renowned Marcelo Bielsa as Head Coach in the summer of 2018. In Bielsa's first game, he scored the third goal in a comprehensive 3–1 victory over promotion favourites Stoke City, as Leeds ascended to the top of the Championship by Christmas.^[23] Leeds would eventually lose out on automatic promotion to Norwich City and Sheffield United as their form dipped in the second half of the 2018–2019 campaign, before losing 4–3 on aggregate to Derby County in the play-offs.^[24] In spite of a disappointing end to a season that had long looked so promising, Cooper was widely perceived to be one of the most improved players in the team under the new head coach, and was included in the EFL Championship Team of the Season, EFL Team of the Season,^[25] as well at the PFA's Championship Team of the Season with team mates Pontus Jansson and Pablo Hernández.^[26]

The 2019–2020 marked Leeds' centenary year and Cooper's sixth season with the club. He would sign a new five year deal with the club in September, and would be present at The Best FIFA Football Awards 2019 to collect the FIFA Fair Play Award on behalf of Bielsa and the Leeds United team for allowing Aston Villa to score an uncontested goal in the previous season.^[27][28] On the pitch, the team's nine–point lead at Christmas in the automatic promotion zone was completely eroded by the beginning of February after a run of four defeats in five games.^[29] With questions again being asked of Leeds’ ability to maintain the form over the course of a full season, Cooper would score a crucial equaliser in a 1–1 draw in the following game against promotion rivals Brentford.^[30] It would later transpire that Cooper's son had been in hospital for three days prior to the game and there were doubts over whether he would play at all.^[31] The result proved to be a catalyst for the Yorkshire club's season, as they went on to win their next five games with Cooper being part of a defence that would also keep five clean sheets, taking the team back to the summit of the table and reestablishing a seven–point lead in the automatic promotion places.^[32] The momentum building around the club with only nine games remaining would be halted six days later when the season was suspended due to the COVID-19 pandemic.^[33]

The season would resume behind closed doors three months later in June, with Leeds' five match winning run coming to an immediate end with a 2–0 defeat to Cardiff City. Leeds would only drop two further points from the remaining eight games, as Leeds secured promotion to the Premier League on 17 July after West Yorkshire rivals Huddersfield Town beat West Bromwich Albion 2–1, and won the Championship title the following day after Brentford failed to beat Stoke City.^[34] Cooper became the first Leeds United captain to win promotion since Gordon Strachan in 1990 and lifted the Championship title on 22 July after a 4–0 win over Charlton Athletic on the final day of the season, immortalising himself in the club's history.^[35] He would also retain his place in the PFA Team of the Year.^[36]

Premier League

Cooper missed Leeds' first Premier League game for 16 years in a 4–3 away defeat to Liverpool on the opening day of the season after picking up an injury. In the following match, he made his first Premier League start since September 2009 in a 4–3 win over Fulham. The period between stands at 10 years, 359 days was a league record for the greatest length of time between starts.^[37] The following game against Sheffield United also marked his 200th appearance with the club and the first player to reach that number of games since Luciano Becchio in 2012.^[37] Cooper joins a select group of just 70 players^[38] that have made over 200 appearances for the club, including current teammate Stuart Dallas who also hit the milestone later in that season.

Return to the Championship

On 6 August 2023, Cooper scored the team's first goal on the opening day of Leeds' 2023–24 Championship league campaign, a 2–2 home draw with Cardiff City, injuring his knee in the process, which kept him out of the team until his 20 September return as a second-half substitute against Hull City.^[39][40]