Rosar had one of his best seasons in the major leagues with Connie Mack's Philadelphia Athletics in 1946, hitting for a .283 batting average and posted career-highs with 120 hits and 48 runs batted in. He led American League catchers in assists, runners caught stealing, and fielding percentage, setting a record for errorless games by a catcher, posting a 1.000 fielding percentage in 117 games played as a catcher. The next year he extended his perfect play to 147 games and was selected to be the starting catcher for the American League in the 1947 All-Star Game. The errorless games record has since been broken by several players.