Charlie, as he was called, made his major league debut at the age of 21 on May 18, 1910, with the Chicago White Sox, who kept him at first base almost exclusively throughout his two seasons with the team. After a three-year absence, Mullen came back to the major leagues in 1914, when he joined the New York Yankees, again playing most games as first baseman.
The slim, right-handed Yankee ended his professional baseball career on September 30, 1916, with a .247 lifetime batting average. In 1919, the Seattle Rainiers hired Mullen to replace manager Bill Clymer. Mullen died in Seattle on June 6, 1963.