Mack's musical career was established during World War II, he enlisted on 4 June 1942 in the RAAF, and was trained as an aircraft mechanic; he was discharged on 7 January 1946 with the rank of corporal from 62 ACW (Airfield Construct Wing). In 1944 whilst serving in Borneo, his ability to play the guitar and sing was noticed, and he was seconded to entertain the troops with visiting guest stars. In May 1946, Mack was an ex-serviceman performer on a theatre concert, who "was responsible for most of the laughs with his vocal gymnastics, his number, 'In Der Fuhrer's Face', being a gem of its kind, which had the audience in hysterics."