Fighting out of Paris, France, Saint-Denis has a background in Judo, being one of many hundreds of thousands to have practiced the style in the 2nd ranked country in Judo. He would later join the French army and ascended the ranks to become a member of The Commandement des Opérations Spéciales, the French militaries special forces branch. 

After retiring from active military service, in MMA competition he would go 7-0 fighting across Europe and once in Bahrain. In BRAVE CF he went a perfect 8-0 before getting signed to the UFC.

He also competed in BJJ, winning the medium-heavyweight (under 88,3 kg/194 lbs) division, in blue belt, at the 2017 West Zone Championship in France, he also participated in the 2017 Paris International Open IBJJF Jiu Jitsu No Gi Championship, in the medium-heavyweight (under 88,3 kg/194 lbs) division, taking 3rd place. He won the middleweight (under 82,3 kg/181 lbs) division, at blue belt, in the absolute category, and took 2nd place on the regular category at the 2018 West Zone Championship in France. He was also the winner of both GI and NO Gi in the 2019 France National Championship, at blue belt in the middleweight (under 82,3 kg/181 lbs) division. He also participated in two shows for ACB JJ notably facing Yusup Raisov, losing by points.

He debuted against Brazilian veteran Elizeu Zaleski Dos Santos in a potential Fight of the Year candidate. Many argue the fight should have been stopped in the 2nd round, with Saint-Denis get lured into a brawl and taking heavy damage, but Saint-Denis would show the French fighting spirit and his special forces level grit, and make it to a decision.