Cronin was born in Excelsior District of San Francisco, California. The 1906 San Francisco earthquake had cost his Irish Catholic parents almost all of their possessions. Cronin attended Sacred Heart High School. He played several sports as a child and he won a city tennis championship for his age group when he was 14.

At the end of the 1947 season, Cronin succeeded Eddie Collins as general manager of the Red Sox and held the post for over 11 years, through mid-January 1959. The Red Sox challenged for the AL pennant in 1948-49 (finishing second by a single game both seasons) thanks to Cronin's aggressive trades. In his first off-season, he acquired shortstop Vern Stephens and pitchers Ellis Kinder and Jack Kramer from the St. Louis Browns; all played major roles in Boston's contending 1948 season, and Kinder and Stephens were centerpieces of the Red Sox' 1949-50 contenders as well.  But the Red Sox began a slow decline during the 1950s and did not seriously contend after 1950. With the exception of Ted Williams (who missed most of the 1952-53 seasons while serving in the Korean War), the 1946-50 core of the team aged quickly and the Red Sox faced a significant rebuilding job starting in 1952. Cronin's acquisition of future American League Most Valuable Player Jackie Jensen from Washington in 1954 represented a coup, but the club misfired on several "bonus babies" who never lived up to their potential.  Most attention has been focused on Cronin and Yawkey's refusal to integrate the Red Sox roster; by January 1959, when Cronin's GM term ended, the Red Sox were the only team in the big leagues without an African-American or Afro-Caribbean player. Notably, Cronin once passed on signing a young Willie Mays and never traded for an African-American player. The Red Sox did not break the baseball color line until six months after Cronin's departure for the AL presidency, when they promoted Pumpsie Green, a utility infielder, from their Triple-A affiliate, the Minneapolis Millers, in July 1959.

What was the Red Sox record while he was general manager?
The Red Sox challenged for the AL pennant in 1948-49