Frank Cassidy served in the Aviation Corps during World War I. He was New York state champion in 1918 and AAU Champion in 1919, representing the Ozanam Club. After his Olympic appearance, Cassidy turned professional, but only fought two known bouts. His first was in December 1920 against Sammy Mosberg, who had won the lightweight gold medal at the 1920 Antwerp Olympics, with Mosberg winning by decision. Cassidy then knocked out Mickey Martell in the second round in August 1922. Cassidy died only 1½ years after the Martell bout which would explain his short career.