In January 2021, Merlin was selected by Nantes manager Raymond Domenech several times in the professional squad.[3][6][7] He eventually made his professional debut for Nantes on 10 February 2021, coming on as a half-time substitute for Ludovic Blas in a 4–2 Coupe de France loss to Lens.[8]
The manager who selected Merlin for the game was not on the bench as the young player entered the field, Domenech having been tested positive for COVID-19, and eventually learning he had been fired right after the Lens game.[9]Antoine Kombouaré succeeded him in the manager position.[10]
On 19 March 2021, Merlin signed his first professional contract with Nantes.[11] On 19 February 2022, he scored his first goal in a 3–1 victory over Paris Saint-Germain.[12] He achieved his first title with the club by winning the Coupe de France.[13]
Marseille
On 26 January 2024, Merlin moved to fellow Ligue 1 side Marseille, having agreed a contract until 2028.[14] The transfer fee paid to Nantes was reported as €12 million.[14] In the latter stages of the 2023–24 season, he became part of the team which reached the Europa League semi-finals.[15]