Newhouser was a schoolboy star at Wilbur Wright High School in his hometown of Detroit, Michigan, signed by the Detroit Tigers in 1939 at the age of 18. He made his debut on September 29 of that year. In 1940, he earned a spot on the Tigers out of spring training and remained with the team until 1953.
In his first two full big-league seasons, the young left-hander was plagued with control problems, walking more batters than he struck out while posting records of 9–9 and 9–11. He improved in 1942 and 1943, posting excellent ERAs, but still losing more than he won on a team with a weak offense.
As World War II got under way, the Tigers moved up in the standings because several of their top players, including Newhouser, were classified as 4-F (ineligible to be drafted). Newhouser was 4-F due to a leaky heart valve; he attempted to join the service anyway but was turned down several times.
He blossomed all at once in 1944, becoming a dominant pitcher in wartime baseball. That season, Newhouser rang up a 29–9 record, leading the league in wins and strikeouts (187). His 2.22 ERA was second in the league, as were his 25 complete games and six shutouts. The Tigers jumped into contention, finishing second in the American League, with Newhouser named MVP. Newhouser won the first Sporting News Pitcher of the Year Award in 1944.
In 1945, he repeated as MVP. This time, he won the pitcher's Triple Crown, leading the American League in wins (25, against nine losses), ERA (1.81) and strikeouts (212). He also led the league in innings pitched, games started, complete games and shutouts. Newhouser pitched four innings of relief on the season's final day as Detroit rallied for the pennant. He then won two games in the World Series to help his team to the World Championship, including the deciding seventh game. Newhouser won the second Pitcher of the Year Award in 1945. He is currently the youngest player to win the award two consecutive years at the age of 24. Newhouser was the Sporting News Player of the Year.
In 1946, he went 26–9 with a 1.94 ERA, again leading the league in wins and ERA. His 275 strikeouts was second in the league. Newhouser was runner-up in the MVP race to Ted Williams. He was denied a third straight Pitcher of the Year Award when they suspended the award in 1946 and 1947.
Newhouser continued to rate among the game's best pitchers for the next five years. He won 17 games in 1947, led the AL with 21 wins in 1948 and rang up an 18–11 mark in 1949. After a 15–13 season in 1950, he hurt his arm and his workload was cut significantly.
After being released by the Tigers following the 1953 season, Newhouser signed on with the Cleveland Indians and was their top long reliever in 1954, when Cleveland won 111 games and the pennant. In his final big-league hurrah, he posted a 7–2 mark with a 2.54 ERA, and got to pitch in his second World Series.
He ended his career with a record of 207–150 and a 3.06 ERA. He is the only pitcher ever to win consecutive MVP awards. In 1992, he was elected to the National Baseball Hall of Fame.
After retirement, Newhouser worked as a scout for the Houston Astros, Baltimore Orioles, Cleveland Indians, and the Detroit Tigers. While with the Astros, Newhouser was credited with discovering Derek Jeter, whom the Astros passed over for Phil Nevin. After the Astros passed on Jeter, he quit his job with the Astros in protest after they ignored his drafting advice.
Newhouser discovered, as a scout with the Orioles, a Detroit high schooler named Milt Pappas, who went on to win 209 games in his career — two more than Newhouser did, and Dean Chance who also had a long baseball career, including winning the Cy Young Award for the Los Angeles Angels.
The Detroit Tigers retired Newhouser's number 16 in 1997, and he died one year later.