Grousset was born and raised in the French sui generis overseas collectivity of New Caledonia, and began swimming at age five.

[6] He competed in the 4×100 metre mixed freestyle relay event at the 2018 European Aquatics Championships, winning the gold medal.

[8][9] Later in the year, he won a bronze medal in the 4×50 metre mixed freestyle relay at the 2019 European Short Course Championships, held in December in Glasgow, Scotland, leading off with a 21.35 to contribute to a finish in a French record time of 1:28.86.

[10] In 2021, at the 2020 Summer Olympics in Tokyo, Japan, he placed fourth in the 100 metre freestyle, with a time of 47.72 seconds, behind Caeleb Dressel of the United States, Kyle Chalmers of Australia, and Kliment Kolesnikov of Russia.

[24][25] Day two of competition, in the morning preliminaries of the 100 metre freestyle, he ranked fourth and qualified for the evening semifinals with a time of 48.31 seconds.

[34][35][36] The next day, he swam the lead-off leg of the 4×100 metre mixed freestyle relay, contributing a 48.02 to the final mark of 3:22.80 to win the gold medal.

[37][38] For the preliminaries of the 50 metre freestyle in the morning on day six, he tied Diogo Ribeiro of Portugal for fourteenth rank with a time of 22.23 seconds and qualified for the semifinals.

[59] In the evening session, he led-off the finals relay with a 20.92 to contribute to a new world record time of 1:27.33 and winning the gold medal.

[69][70] At the 2023 Camille Muffat Meeting in March in Nice, Grousset won the silver medal in the 100 metre freestyle with a 48.97, finishing 0.02 seconds behind Szebasztián Szabó of Hungary.

[72][74] On 11 June, day one of the 2023 French Elite Swimming Championships in Rennes, he won the gold medal in the 50 metre butterfly with a time of 23.06 seconds.