Born in Tours, Diallo joined the youth academy of AS Monaco FC at the age of 15. On 28 March 2014, he signed his first professional contract with the club. Vice-president of the club, Vadim Vasilyev commented that Diallo "fits perfectly into our sports project. He has a lot of talent and we hope that he will continue to progress alongside the great players of AS Monaco". On 14 December, he made his first team debut, coming on as a substitute for Bernardo Silva in the extra time of a 1–0 league victory against Olympique Marseille. In June 2015, Diallo was loaned out to Belgian club Zulte Waregem for the 2015–16 season. During his stint at the club, he was deployed at an attacking role, scoring three goals in 33 league matches. In December 2016, media reports emerged that Spanish club Real Betis was interested in signing him. Nevertheless, he made five league appearances during the 2016–17 season, with his side winning the league.
Mainz 05
On 14 July 2017, Diallo moved to Bundesliga club 1. FSV Mainz 05, and signed a five-year contract. On 9 September, he scored his first goal for the club in a 3–1 league victory against Bayer 04 Leverkusen. In December, Mirror.co.uk reported that English club Arsenal was scouting him in a bid to replace the retiring Per Mertesacker. During the season, he was deployed at both back three and back four; and had an 80% pass success rate. He started in 27 league matches during the season.
On 26 June 2018, Diallo joined Borussia Dortmund on a five-year deal for a fee of €28 million. In an interview, he said that Ousmane Dembélé advised him to join the club. On 15 September, he scored his first goal for the club in a 3–1 league victory against Eintracht Frankfurt.
Paris Saint-Germain
On 16 July2019, Diallo signed for Paris Saint-Germain until June 2024. The transfer fee of his deal to the French club was of €32 million. On 3 August 2019, Diallo made his debut for Paris Saint-Germain in the Trophée des Champions, which ended as a 2–1 win for PSG over Rennes. He made his league debut against Nîmes, in a 3–0 home win on 11 August 2019. Diallo's Champions League debut came on 18 September 2019, in a 3–0 win against Real Madrid. At the end of the 2019–20 season, Diallo received a Ligue 1 winners medal for his contributions that season, which was ended early due to the COVID-19 pandemic.
International career
Diallo was born in France and is of Senegalese descent. He captained France's under-21s.
Style of play
Diallo plays as a central defender. Rouven Schröder has said that Diallo is "strong in the air and shrewd in the challenge". Michael Zorc, the sporting director of Borussia Dortmund described Diallo as "a modern, strong central defender who is very intelligent. He can play a wider defensive role too or even be deployed in a defensive midfield role".
