Mario Gentili was an all-round cyclist. In his early years, he was mostly a track cyclist, and at the 1936 Berlin Olympics won a silver medal in the 4,000 metres team pursuit with Bianco Bianchi, Armando Latini and Severino Rigoni, setting an Olympic record on their way to the final. In the years following the Olympics, Gentili struggled to find success but it was only after turning to cyclo-cross in 1940, that he again found the winning formula. He was Italian cyclo-cross champion in 1942, and the following year won the Coppa Del Vasto. Gentili retired from cycling in 1945.