A consistent and durable pitcher, Walberg averaged 16 wins for the Philadelphia Athletics of Connie Mack from 1926 to 1932, with career-highs of 20 wins in 1931 and 18 in 1929. He also had a 1–1 mark with a 1.93 ERA for the Athletics in five World Series appearances. A good-hitting pitcher, Walberg collected a .220 batting average with four home runs and 84 runs batted in. When Mack dismantled the Athletics in 1933, he was sent along with Lefty Grove and Max Bishop to the Boston Red Sox in exchange for two players and $150.000. He was a spot starter and reliever with Boston during three seasons and pitched his last game at the age of forty-one.
Walberg surrendered 17 home runs to Babe Ruth, more than did any other pitcher.