She subsequently qualified as a nurse, living in Broken Hill, New South Wales, for a period and then returning to Adelaide in 1924.
McCarthy joined the South Australia Police in 1929, inspired by Kate Cocks; she was its twelfth female recruit.
As with the other women on the force, she was primarily tasked with handling female prisoners, suspects, and witnesses.
She learned ju-jitsu and was issued with a pistol, but only rarely made arrests; she was not required to wear uniform.
McCarthy retired from the police in 1960, and subsequently worked as a welfare officer for the Supreme Court of South Australia.