Mann played in Major League Baseball (MLB) as an outfielder from 1913 to 1928 for seven teams, spending eight years of his career with the Boston Braves.

He helped bring baseball to the 1936 Berlin Olympics and coached the United States national team at the inaugural 1938 Amateur World Series.

Mann was the head basketball coach at Rice Institute (1919–1920), Indiana University (1922–1924), and Springfield College (1924–1926).

From 1913 to 1928, he played for the Boston Braves, St. Louis Cardinals, Cincinnati Reds, New York Giants, and Chicago Cubs.

[3] The team then went on to defeat Connie Mack's heavily favored Philadelphia Athletics in the 1914 World Series.

As a member of the pennant winning Cubs in 1918, Mann also recorded an RBI single off Babe Ruth in Game 4 of the 1918 World Series.

Mann had a reputation as a "clean" player, who disapproved of vices like drinking and gambling in the clubhouse.

Douglas, at odds with Giants manager John McGraw, suggested that he would be willing to jump ship, effectively tipping the pennant race to the Cardinals.

He organized a 12-game tour of Japan in 1935, with an American amateur team taking eight games and dropping four against Japanese opponents.

[10] Baseball's inclusion in the Olympics was opposed by powerful figures like Avery Brundage, head of the USOC (and future president of the International Olympic Committee), who was "a firm believer in the idea that there was no such thing as an amateur baseball player.

Mann also had hopes to establish Olympic baseball at the 1944 Games in London, but those too were canceled due to the outbreak of war.