Major League CareerEditBaseball promoter Joe Engel, who scouted for the Senators and managed the Chattanooga Lookouts at Engel Stadium, originally signed Cronin. When Engel first spotted Cronin playing in Kansas City, "I knew I was watching a great player. I bought Cronin at a time he was hitting .221. When I told Clark Griffith what I had done, he screamed, 'You paid $7,500 for that bum? Well, you didn't buy him for me. You bought him for yourself. He's not my ballplayer - he's yours. You keep him and don't either you or Cronin show up at the ballpark.'" Cronin led the Senators to the 1933 World Series and later married Griffith's niece, Mildred Robertson.
In 1930, Cronin had a breakout year, batting .346 with 13 home runs and 126 RBI. Cronin won both the AL Writers' MVP and the AL Sporting News MVP. His 1931 season was also outstanding, posting a .306 average, 12 home runs and 126 RBI.
Cronin assumed managerial duties in 1933, a role he would continue with the Boston Red Sox, the team he was traded to prior to the 1935 season, by Senators' owner Clark Griffith. Cronin retired as a player in 1945 (though he was only a part-time player after 1941), but remained as manager of the Red Sox until 1947.
Over his career, Cronin batted .300 or higher eight times as well as knocking in 100 runs or more eight times. He finished with a .301 average, 170 home runs and 1424 RBI. As a manager, he compiled a 1,236-1,055 record and won two American League championships (in 1933 and 1946). His 1933 Senators dropped the 1933 World Series to the New York Giants, and his 1946 Boston Red Sox lost the 1946 World Series to the St. Louis Cardinals.
At the end of the 1947 season, Cronin succeeded Eddie Collins as general manager of the Red Sox and continued through mid-January 1959. The Red Sox competed for the AL pennant in 1948-49 (finishing second by a single game both seasons) thanks to Cronin's aggressive trades, but they began a slow decline during the 1950s and did not seriously contend until after 1950. While most attention has been focused on the Red Sox being the last major league team to integrate its roster (Cronin once passed on signing a young Willie Mays and never traded for an African-American player during his GM tenure), another factor was a large number of "bonus babies" who never panned out.
In January 1959, Cronin was elected president of the American League, the first former player to be so elected. Cronin served as American League president until the end of 1973, when he was succeeded by Lee MacPhail.
He was inducted into the Baseball Hall of Fame in 1956. His jersey number 4 was formally retired by the Red Sox on May 29, 1984. In 1999, he was named as a finalist to the Major League Baseball All-Century Team.