After spending time with the youth team and reserve team, Grandsir signed with the Troyes senior team in 2016. Garnering attention from other clubs after several impressive seasons, he moved to
Monaco for €5.250m in 2018. Grandsir went on loan from Monaco to
Strasbourg in January 2019 followed by another loan to
Brest for the 2019–20 season. On 11 March 2021, Grandsir signed with
Major League Soccer club
LA Galaxy. On 23 January 2023, Grandsir returned to his native France to join
Ligue 2 leaders
Le Havre on a contract until the end of the season. On 2 February 2025, Grandsir signed with
Caen in
Ligue 2. ==International career==