On 3 March 2020, Assignon signed his first professional contract with Rennes.[4] During the second half of the 2020–21 season, he was loaned to third-tier Bastia, where he helped the club win the Championnat National, contributing one goal in thirteen appearances.[3]
Assignon made his professional debut for Rennes as a substitute in a 3–1 UEFA Europa Conference League win over Rosenborg on 26 August 2021.[5] On 8 September 2022, he scored his first professional goal for Rennes, a last-minute winner in a 2–1 UEFA Europa League victory over AEK Larnaca. Sports newspaper L'Équipe described his goal as "Zlatan-esque".[6]
Burnley (loan)
On 1 February 2024, Assignon was loaned out to English side Burnley until the end of the season with an option to buy.[7] On 30 March 2024, Assignon gave away a penalty and got a red card after receiving his second booking for a foul on Mykhailo Mudryk, with the game against Chelsea ending in a 2-2 draw. After his second yellow card, Vincent Kompany was also sent off for protesting against the referee Darren England over awarding the home side the penalty.[8] On 20 April 2024 he became just the second Burnley defender to both score and assist in a single Premier League game, in a 4-1 away win at Sheffield United.[9] On 21 May 2024, Burnley said it was moving to make the loan permanent.[10]