Born in Nice, Sarr joined local side OGC Nice at the age of six. He spent the next 12 years in the club's academy before making his Ligue 1 debut for the club on 14 August 2016 against Rennes, aged 17. He scored the winning goal for his team in the 60th minute, heading home from a Jean-Michaël Seri free-kick. In doing so, he became the second youngest player to ever score on debut in Ligue 1, behind Nigerian international Bartholomew Ogbeche. After the match, Sarr dedicated his goal to the victims of the 2016 Nice terror attack. Prior to kick-off a minute's silence was observed in memory of the victims with both Nice and Rennes wearing unique commemorative shirts bearing the names of those who had died. On 7 November 2016, Sarr signed his first professional contract with Nice and by the end of the season had made over 30 appearances as the club ended the league campaign in third place.