He spent eight years there, attracting the interest of Europe's elite clubs including Arsenal, Chelsea, Manchester City, and Bayern Munich.

[7] He made his professional debut on 28 October 2014, playing only 79 minutes at right-back as Chelsea won 2–1 away to Shrewsbury Town in the fourth round of the League Cup.

[8] Christensen did not play again until 24 January 2015, when he was utilised in the same position for Chelsea's 4–2 home defeat to League One team Bradford City in the FA Cup fourth round.

[9] Although Christensen played no further part in the campaign, Chelsea won the League Cup with a 2–0 victory over rival club Tottenham Hotspur in the final.

[19] After the success of his first season, Mönchengladbach made multiple attempts to make his loan permanent during the 2016 summer transfer window, although it was reported that Chelsea rejected the £14.25 million bid from the German club.

[27] New manager Maurizio Sarri preferred to pair David Luiz and Antonio Rüdiger in central defence, and by February 2019 Christensen had made 15 appearances, only two of which in the league.

[30] On 4 July 2022, Christensen signed for La Liga club Barcelona in a free transfer on a four-year contract with a buyout clause of €500 million.

[32] He began playing more consistently as the season progressed, and eventually formed a defensive partnership alongside Ronald Araújo, helping Barcelona keep the most clean sheets (16) in all of Europe's top 5 leagues.

[35] Following poor performances from teammate Oriol Romeu and poor general defensive performances, culminating in narrow wins and a shock 3–5 home defeat to Villarreal, Xavi moved Christensen to defensive midfielder, which managed to improve Barcelona's form, including emphatic victories against Napoli in the Champions League and rivals Atlético Madrid.

On 10 April 2024, he scored his first Champions League goal with Barcelona, after coming off the bench in the second half, which granted his club a 3–2 away win against Paris Saint-Germain in the quarter-final first leg.

[36] On 8 June 2015, Christensen made his senior international debut for Denmark in a 2–1 home friendly win against Montenegro at the Viborg Stadium, as a 69th-minute substitute for Pierre-Emile Højbjerg.

[43] On 21 June 2021, Christensen scored a thunderous goal from long range against Russia in a 4–1 win that helped Denmark advance to the knockout-phase of the tournament.