Magni began his career with Varese in 1937 and made 53 appearances in Serie C over the next three years, scoring nine goals. He joined Serie B club Liguria in 1940 – who would later become Sampdoria – and helped the club win promotion to Serie A in his first season, scoring 11 goals in 31 games. Having scored 12 times in 45 games for Liguria, Magni was transferred to Juventus in 1942. He scored ten league goals in his first season with Juventus, including four in one match – a 6–2 win against Triestina. He spent six years with the club, but only took part in four league campaigns due to the Second World War, which cancelled competitive football in Italy for two years. Magni scored 27 goals in 106 games for Juventus.