His greatest success Leoni at the Giro d' Italia, which he finished in fourth place in 1949, during which he wore the Maglia Rosa eight days. Overall, he won 17 Giro stages. His only participation in the Tour de France ended involuntarily after the eleventh stage. On this were the torn Gino Bartali and Jean Robic by drunken fans on the Col d' Aspin brought down and Bartali was subsequently greatly inconvenienced. Although Bartali won the stage, but in protest to the Italian team withdrew. Previously Leoni had won the second stage. In 1937, Leoni was able to win the world amateur championship.
- Tour de France 1950: Stage 2
- Giro d' Italia 1938: Stage 6; 1939: Stage 7; 1940: 4th, 5th, 12th and 20th stage; 1946: Stage 3; 1947: 14th, 17th and 19th stage; 1948: 5th and 8th stage; 1949: 7th, 9th and 13th stage; 1950: Stage 11; 1951: Stage 14
- Milan - Sanremo: 1942