Johnson won renown as the premier power pitcher of his era. Although a lack of precision instruments prevented accurate measurement of his fastball, Johnson is believed to have thrown as high as 99 miles per hour from a sidearm angle. This power is exceptional even today, but it was virtually unique in Johnson's day. The overpowering fastball is the primary reason for Johnson's exceptional statistics, especially his strikeout totals.

Nicknamed Big Train, as a right-handed pitcher for the Washington Nationals/Senators, he won 417 games, the second most by any pitcher in history (after Cy Young, who won 511). He and Young are the only pitchers to have won 400 games or more. In a twenty-one year career he had twelve twenty-win seasons, including ten in a row, and two seasons in which he had more than thirty wins (33 in 1912 and 36 in 1913). His record includes 110 shutouts, the most in baseball history. He thrice won the triple crown for pitchers (1913, 1918, 1924) and twice won the American League Most Valuable Player Award (1913, 1924). On September 4, 5, and 7, 1908, he shut out the New York Yankees in three consecutive games. His 3,508 all-time strikeout record stood for 56 years, until Steve Carlton, Nolan Ryan, and Gaylord Perry (in that order) broke it in 1983. His earned run average of 1.14 in 1913 set a record that stood until Bob Gibson's 1.12 ERA in 1968.

That 1.14 ERA in 1913 should 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. The farce game in 1913 saw the 43 year old Griffith playing a ball off his head while playing Right Field. He also misplayed a liner that resulted in an inside-the-park homer, and he also was one of eight pitchers to appear in the game. Griffith played Walter Johnson in Center Field until bringing him in to pitch in the 8th inning. In that appearance, Johnson lobbed pitches to two hitters that resulted in hits before he was sent back to playing Center Field. The subsequent pitcher (actually a Catcher making his only Major League pitching appearance) then allowed the two runners to score. Some record books still indicate that Johnson had a 1.09 ERA for 1913. The official scorekeeper ignored the game, but later, Johnson was charged with those two runs, raising his ERA and allowing Gibson to break the record.

Although he usually pitched for losing teams during his career, Johnson led the Nationals/Senators to two World Series, a victory in 1924 (including the final, 12-inning game) and a loss in 1925. Johnson was a better-than-average hitter for a pitcher, compiling a career batting average of .235. He also made 13 appearances in the outfield during his career.

In 1928, he began his career as a manager in the minor leagues, taking up residence at 32 Maple Terrace, Millburn, New Jersey, and managing the Newark team of the International League. He continued on to the major leagues, managing the Nationals/Senators (1929-1932), and finally the Cleveland Indians (1933-1935).

One of the first five electees to the Baseball Hall of Fame in 1936, Walter Johnson retired in Germantown, Maryland and was elected Montgomery County commissioner in 1938. He lost a very close election to the U.S. Congress in 1940 and died of a brain tumor in Washington, D.C. on December 10, 1946. He is interred in the Rockville Union Cemetery in Rockville, Maryland.

A high school in Bethesda, Maryland has been named for him. (See Walter Johnson High School.) The monument to him that once stood outside Griffith Stadium has been moved to the school's campus.

He was also called Sir Walter and the White Knight because of his gentlemanly gamemanship, and "Old Barney" later in his career. In 1995, the rock musician Jonathan Richman recorded a song entitled "Walter Johnson" that celebrated Johnson's kindness.

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

Johnson's gentle nature was legendary, and to this day he is held up as an example of good sportsmanship and his name has become synonymous with friendly competition. This attribute worked to Johnson's disadvantage in the case of fellow Hall of Famer Ty Cobb. Virtually all batters were concerned about being hit by Johnson's fastball, and many would not "dig in" at the plate because of that concern. Cobb realized that the good-hearted Johnson was privately nervous about the possibility of seriously injuring a batsman. Almost alone among his peers, Cobb would actually stand closer to the plate than usual when facing Johnson.

Johnson's rookie season was Cobb's third, and Johnson retired one year before Cobb. Cobb faced Johnson at bat more times in their overlapping careers than any other hitter-pitcher combination in major league history.