Walter's big break was when he moved to Idaho, where he doubled as a telephone company employee and a pitcher for a team in Weiser, Idaho of the Idaho State League.
He played his entire 21-year baseball career for the Washington Senators (1907–1927). He later served as manager of the Senators from 1929 through 1932 and of the Cleveland Indians from 1933 through 1935.
Often thought of as one of the greatest pitchers in baseball history, Johnson established several pitching records, some of which remain unbroken nine decades after he retired from baseball. He remains by far the all-time career leader in shutouts with 110, second in wins with 417, and fourth in complete games with 531. He held the career record in strikeouts for nearly 56 years, with 3,508, from the end of his career in 1927 until the 1983 season, when three players (Steve Carlton, Nolan Ryan and Gaylord Perry) finally passed the mark. Johnson was the only player in the 3,000 strikeout club for 51 years until Bob Gibson recorded his 3,000th strikeout. Johnson led the league in strikeouts a Major League record 12 times—one more than current strikeout leader Nolan Ryan—including a record eight consecutive seasons. He is the only pitcher in major league history to record over 400 wins and strike out over 3,500 batters.
In 1936 he was one of the first five players elected to the Baseball Hall of Fame in 1936. Johnson, Ty Cobb, Christy Mathewson, Babe Ruth and Honus Wagner were known as the "Five Immortals" because they were the first players chosen for the Baseball Hall of Fame.