Foni was born in Udine, and he made his professional debut with local club Udinese at the age of 16, in 1927, in the Prima Divisione (the predecessor to the Italian Serie A); he later moved to Lazio in 1929. He later moved to Padova for a brief stint after a few seasons in Roma.
In total, he made 266 League appearances with Juventus, and 370 total appearances in Serie A, after making his debut in the competition on 2 February 1930, with Lazio, against Pro Vercelli. He moved to play with Chiasso during the 1948-49 season, making only three appearances in the league, before retiring.
He became a permanent member of the starting line-up at the 1938 FIFA World Cup, replacing Roma full-back Eraldo Monzeglio, starting along-side his Juventus team-mate Rava, and helping Italy to defend their World Cup title. He ended his international career in 1942, with 23 appearances; along with Pietro Rava, Sergio Bertoni, and Ugo Locatelli, Foni is one of only four Italian players ever to win both the Olympic tournament and the World Cup.