Mamadou Sarr was born on 29 August 2005 in Martigues, France, the son of former Senegalese international Pape Sarr. He grew up in Nord-Pas-de-Calais before beginning his academy career at Étoile Sportive de Saint-Laurent-Blangy and then Lens, where his father played. He joined Lyon in 2018 and progressed steadily through their youth ranks.
At just 16, Sarr established himself as a starter in Lyon's under-19 side, playing a key role in their 2022 Coupe Gambardella triumph — scoring in both the quarter-final against Strasbourg and the semi-final against Troyes. He made his professional debut for Lyon in May 2023, coming on as a substitute in a 3–0 Ligue 1 win over Reims. After a loan spell at RWD Molenbeek, he joined Strasbourg permanently in August 2024 for €10 million. On the international stage, he won the UEFA European Under-17 Championship with France in 2022, before switching allegiance to Senegal in December 2025. He started the full 120 minutes of the 2025 Africa Cup of Nations final, helping Senegal claim the title against Morocco.
A composed centre-back who can also operate as a defensive midfielder, Sarr signed an eight-year contract with Chelsea in June 2025 and, after a loan back to Strasbourg, was recalled in February 2026 and made his first start for the club in a 4–0 FA Cup win over Hull City. Chelsea view him as a long-term defensive cornerstone.