Giacomo Mari had a long career in the Italian leagues, starting in 1942 with Cremonese, and then playing with Atalanta Bergamasca, Juventus FC, UC Sampdoria, Calcio Padova, and finishing his career in 1960/61 back with Cremonese. Mari won two Italian Championships with Juventus in 1949/50 and 1951/52. Mari débuted with the Italian team at the 1948 Olympics, and eventually earned eight international caps. He played at both the 1950 and 1954 World Cups, one of five Italian players to do so (also Giampiero Bonpierti, Gino Cappello, Ermes Muccinelli, and Egisto Pandolfini). He later managed Calcio Padova in 1961/62.