His baseball career was interrupted by his military service in the United States Army during World War II.

At the time of his retirement in 1965, Spahn held the Major League record for career strikeouts by a left-handed pitcher.

[1] The Warren Spahn Award, given annually to the major leagues' best left-handed pitcher, is named in his honor.

[4] Regarded as a "thinking man's" pitcher who liked to outwit batters, Spahn once described his approach on the mound: "Hitting is timing.

Reluctantly, he took up pitching and led his high school team to two city championships, going undefeated his last two seasons, and throwing a no-hitter his senior year.

[6] Spahn's major league career began in 1942 with the Braves organization and he spent all but one year with that franchise, first in Boston and then in Milwaukee.

Spahn made his professional debut on July 6 at MacArthur Park (Dwyer Stadium) in Batavia, New York.

In 1941, Spahn broke out and won 19 games against 6 losses with a 1.83 ERA while pitching for the Class-B Evansville Bees of the Illinois-Indiana-Iowa League.

He clashed with Braves manager Casey Stengel, who sent him back to the minors after Spahn refused to throw at Brooklyn Dodgers batter Pee Wee Reese in an exhibition game.

[3] Stengel later said that it was the worst managing mistake he had ever made: "I said "no guts" to a kid who went on to become a war hero and one of the greatest lefthanded pitchers you ever saw.

[8] Along with many other major leaguers, Spahn chose to enlist in the United States Army, after finishing the 1942 season in the minors.

[12] Spahn was unsure of the war's impact on his career: People say that my absence from the big leagues may have cost me a chance to win 400 games.

[13]Spahn's first full season as a starting pitcher came in 1947, when he led the National League in ERA (2.33), shutouts (7), and innings pitched (289+2⁄3) while posting a 21–10 record.

The team swept a Labor Day doubleheader, with Spahn throwing a complete 14-inning win in the opener, and Sain pitching a shutout in the second game.

For example, some referred to the 1993 San Francisco Giants' imbalanced rotation as "Burkett and Swift and pray for snow drift.

[20] On July 2, 1963, facing the San Francisco Giants, the 42-year-old Spahn became locked into a storied pitchers' duel with 25-year-old Juan Marichal.

[21] The score was still 0–0 after more than four hours when Willie Mays hit a game-winning solo home run off Spahn with one out in the bottom of the 16th inning.

Marichal ended up throwing 227 pitches in the complete game 1–0 win, while Spahn threw 201 in the loss, allowing nine hits and one walk.

[3] Braves manager Bobby Bragan predicted, "Spahnie won't win six games with the Mets."

[25] He signed with the San Francisco Giants, with whom he appeared in his final major league game on October 1, 1965, at the age of 44.

[3] In a 22-season major league career, Spahn posted a 363–245 win–loss record with 2,583 strikeouts and a 3.09 ERA in 5,243.2 innings pitched, including 63 shutouts and 382 complete games.

[3] His 2,583 career strikeouts were the most by a left-handed pitcher in Major League Baseball history until he was later on surpassed by Mickey Lolich in 1975.

He was a pitching coach with the Cleveland Indians, in the minor leagues for the California Angels, and for six years, with Japan's Hiroshima Toyo Carp.

For many years he owned and ran the large Diamond Star Ranch south of Hartshorne, Oklahoma, before retiring to live near a golf course in Broken Arrow with his half-Cherokee wife LoRene (née Southard) with whom he had one child, a son named Gregory (1948–2022).

[28] He was selected for the all-time All-Star baseball team by Sports Illustrated magazine in 1991, as the left-handed pitcher.

The trio, who were Milwaukee-area natives, included Spahn in the closing credits once, with the disclaimer, "He's not in the film, but he's still our all-time favorite left-hand pitcher."

Spahn also made his acting debut with a cameo appearance as a German soldier in a 1963 episode (S2E8 "Glow Against the Sky")[36] of the television series Combat!

In 2013, the Bob Feller Act of Valor Award honored Spahn as one of 37 Baseball Hall of Fame members for his service in the United States Army during World War II.