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 footballing education at Étoile Sportive de Saint-Laurent-Blangy and then Lens, the club where his father spent much of his career. In 2018 he moved to Lyon's academy, and by the 2021–22 season he had established himself as a regular starter for their under-19 side, contributing goals in both the quarter-final and semi-final as Lyon won the Coupe Gambardella.
Sarr made his professional debut for Lyon in May 2023 and joined Strasbourg on a €10 million transfer in August 2024. He signed with Chelsea on an eight-year contract in June 2025, earning a Club World Cup winner's medal with the club that same year. After a loan spell back at Strasbourg, he was recalled by Chelsea in February 2026 and made his first start for the club in a 4–0 FA Cup win over Hull City.
Internationally, Sarr represented France at youth level, including the 2022 UEFA European Under-17 Championship, which France won. He subsequently switched allegiance to Senegal, with FIFA approving the change in December 2025. He earned three caps at the 2025 Africa Cup of Nations, starting the final against Morocco as Senegal claimed the title — their second AFCON trophy in history.
A centre-back by trade, Sarr is a composed, physically assured defender who can also operate in defensive midfield. Chelsea see him as a long-term cornerstone of their backline.