Thomas had three seasons as the primary catcher of the Red Sox, helping them to pennants in 1915-16. He averaged 22 RBI a year as a regular, just over 200 at-bats a season. He led AL catchers in fielding in 1917. After the season, he went to the Athletics with two others and cash in the deal that brought Joe Bush, Wally Schang, and Amos Strunk to the Red Sox; Thomas was sold to the Indians and played on another pennant winner in 1920. From 1913 to 1918 he hit .417 as a pinch hitter (13-for-31), but he failed to get a hit in 16 subsequent pinch at-bats.