In 1915, the year after the Boston Braves swept the Athletics in the World Series, Red Sox owner Joe Lannin paid $8,000 for Barry's services, as Mack was dismantling the team. Upon joining the Red Sox, he hit just .262 but played reliable defense at shortstop, proving to be the last piece of the puzzle in what was to be another pennant-winning team. He played in the World Series in 1915 and 1916 for the Red Sox. Acknowledged as the team's on-field leader, he became a player-manager in 1917, leading the team to a 90-win season and a second-place finish to the Chicago White Sox. In the war year of 1917, manager Jack Barry chose to enlist and on October 18, 1917 Jack and four other Red Sox players, who had enlisted as yeomen in the naval reserve, were called to active duty and ordered to report for duty on November 3, 1917. He served all of 1918 in the military. After poor play in 1919, he decided to retire rather than be sold away in another fire sale following Harry Frazee's decision to sell his shortstop back to the Athletics.