Finding success in Rochester, Kelly was purchased by the Giants in 1919 when Hal Chase was suspended. He became a regular in the Giants line-up in 1920, when he had a league-leading 94 runs batted in (RBIs). He led the league in assists and putouts in 1920 and 1921; his 1,759 putouts in 1920 remains a league record. Kelly batted in 100 or more runs for four consecutive seasons and hit for a batting average of .300 or higher six consecutive seasons. He opened the 1921 season with at least one hit and one RBI in eight consecutive games, a record that stood until it was broken by Jorge Cantú in 2010. The Giants appeared in the World Series in 1921, 1922, 1923, and 1924, winning in 1921 and 1922. Kelly set a National League record with seven home runs in six consecutive games in 1924, which has since been matched by Graig Nettles, Walker Cooper, and Willie Mays.