He threw and batted right-handed; during his playing days, Mulcahy stood 6 feet 2 inches (1.88 m) tall, weighing 190 pounds (86 kg).

Mulcahy had the nickname of "Losing Pitcher";[3] because he lost 20 games in 1938 and 22 in 1940 and never had a full season in the majors in which he recorded more wins than losses.

Mulcahy also gained national distinction when he became the very first major leaguer to be drafted into United States military service before the U.S. entered World War II, on March 8, 1941.

Mulcahy led the NL in earned runs allowed (125), wild pitches (11), and hit batsmen (11) in 1939.

In retirement, Mulcahy spent many years as a scout for the Chicago White Sox, and served one season (1970) as their pitching coach.