Soon after he reached his fourteenth birthday, his family moved to California’s Orange County in 1902. The Johnsons settled in the town of Olinda, a small oil boomtown located just east of Brea. In his youth, Johnson split his time among playing baseball, working in the nearby oil fields, and going horseback riding. Johnson later attended Fullerton Union High School where he struck out 27 batters during a 15-inning game against Santa Ana High School. He later 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. Johnson was spotted by a talent scout and signed a contract with the Washington Senators in July 1907 at the age of 19.

In a 21-year career, Johnson had twelve 20-win seasons, including ten in a row. Twice, he topped 30 wins (33 in 1912 and 36 in 1913). Johnson’s record includes 110 shutouts, the most in baseball history. Johnson had a 38–26 record in games decided by a 1–0 score; both his wins and losses in these games are major league records. Johnson also lost 65 games because his teams failed to score a run. On September 4, 5 and 7, 1908, he shut out the New York Highlanders in three consecutive games.

Johnson’s Hall of Fame plaque reads that he pitched “for many years with a losing team.” While the Senators had only nine winning seasons during his career, they finished in the first division (i. e., fourth place or higher) 11 times, and the second division 10 times. In Johnson’s first five seasons, Washington finished last twice and next-to-last three times. But they finished second in the American League in both 1912 and 1913, which were Johnson’s two 30-win seasons. Then, for the next decade, they typically finished in the middle of the pack before their back-to-back pennants.

His earned run average of 1.14 in 1913 was the fourth-lowest ever at the time he recorded it; it remains the sixth-lowest today, despite having been surpassed by Bob Gibson in 1968 (1.12) for lowest ERA ever by a 300+ inning pitcher. It could have been lower if not for one of manager Clark Griffith’s traditions. For the last game of the season, Griffith often treated the fans to a farce game. Johnson actually played center field that game until he was brought in to pitch. He allowed two hits before he was taken out of the game. The next pitcher—who was actually a career catcher—allowed both runners to score. The official scorekeeper ignored the game, but later, Johnson was charged with those two runs, raising his ERA from 1.09 to 1.14. For the decade from 1910–1919, Johnson averaged 26 wins per season and had an overall ERA of 1.59.

Johnson won 36 games in 1913, 40% of the team’s total wins for the season. In April and May, he pitched 55.2 consecutive scoreless innings, still the American League record and the third-longest streak in history. In May 1918, Johnson pitched 40 consecutive scoreless innings; he is the only pitcher with two such 40+ inning streaks.

In 1917, a Bridgeport, Connecticut munitions laboratory recorded Johnson’s fastball at 134 feet per second, which is equal to 91 miles per hour (146 km/h), a velocity that may have been unmatched in his day, with the possible exception of Smoky Joe Wood. Johnson, moreover, pitched with a sidearm motion, whereas power pitchers are usually known for pitching with a straight-overhand delivery. Johnson’s motion was especially difficult for right-handed batters to follow, as the ball seemed to be coming from third base. His pitching mechanics were superb, generating powerful rotation of his shoulders with excellent balance. In addition to his fastball, Johnson featured an occasional curveball that he developed around 1913 or 1914. He batted and threw right-handed.

Although he often pitched for losing teams during his career, Johnson finally led the Washington Senators to the World Series in 1924, his 18th year in the American League. Johnson lost the first and fifth games of the 1924 World Series, but became the hero by pitching four scoreless innings of relief in the seventh and deciding game, winning in the 12th inning. Washington returned to the World Series the following season, but Johnson’s experience was close to the inverse: two early wins, followed by a game seven loss. On October 15, 1927, Johnson’s request for an unconditional release from the club was granted.

Johnson was a good hitter for a pitcher, compiling a career batting average of .235, including a record .433 average in 1925. He also made 13 appearances in the outfield during his career. He hit over .200 in 13 of his 21 seasons, hit three home runs in 1914, and hit 12 doubles and a triple in 130 at-bats in 1917. Johnson finished his career with 23 home runs (he hit a pinch-hit home run in 1925) and 24 overall, the ninth-highest total for a pitcher in Major League history.

In 1928, he began his career as a manager in the minor leagues, managing the Newark Bears of the International League. He continued on to the major leagues, managing the Washington Senators (1929–1932), and finally the Cleveland Indians (1933–1935). His managing record was 529–432, with his best team managed being in 1930, when the team finished 94–60, 8 games out of first place. In seven seasons, he had five winning seasons, with the only two losing seasons being at the beginning of his tenure with Washington and Cleveland, though his teams did not come close to winning the pennant, finishing 12 games behind in his last season. Johnson also served as a radio announcer on station WJSV for the Senators during the 1939 season.

Walter married Hazel Lee Roberts June 24, 1914, and they had five children. His wife died in August 1930 from complications resulting from heat stroke after a long train ride from Kansas.

Three times, Johnson won the triple crown for pitchers (1913, 1918 and 1924). Johnson twice won the American League Most Valuable Player Award (1913, 1924), a feat accomplished since by only two other pitchers, Carl Hubbell in 1933 and 1936 and Hal Newhouser in 1944 and 1945.

The overpowering fastball was the primary reason for Johnson’s exceptional statistics, especially his fabled strikeout totals. Johnson’s record total of 3,508 strikeouts stood for more than 55 years until Nolan Ryan, Steve Carlton, and Gaylord Perry all surpassed it in that order during the 1983 season. Johnson, as of 2017, ranks ninth on the all-time strikeout list, but his total must be understood in its proper context of an era of much fewer strikeouts. Among his pre–World War II contemporaries, only two men finished within 1,000 strikeouts of Johnson: runner-up Cy Young with 2,803 (705 strikeouts behind) and Tim Keefe at 2,562 (946 behind). Bob Feller, whose war-shortened career began in 1936, later ended up with 2,581.

Johnson 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.

Walter Johnson retired to Germantown, Maryland. A lifelong Republican and friend of President Calvin Coolidge, Johnson was elected as a Montgomery County commissioner in 1938. His father-in-law was Rep. Edwin Roberts, a Republican member of the U.S. House of Representatives. In 1940 Johnson ran for a congressional seat in Maryland’s 6th district, but came up short against the incumbent Democrat, William D. Byron, by a total of 60,037 (53%) to 52,258 (47%).

In 1985, Jonathan Richman recorded the song “Walter Johnson”, which dwelt on Johnson’s personality and behaviour as an exemplar of what can be good in sport.

In 1999, The Sporting News ranked Johnson number 4 on its list of Baseball’s 100 Greatest Players, the highest-ranked pitcher. Later that year, he was elected to the Major League Baseball All-Century Team.

In 2015, he along with Nap Lajoie, Christy Mathewson and Cy Young were named the “Greatest Pioneers Group.” They were voted for by baseball fans online as part of the Franchise Four competition and were “selected as the most impactful players”. The results were announced at the 2015 MLB All-Star Game.