Unable to secure a teaching position due to the Depression, McCarthy found work as a patrol officer in New Guinea where his area of responsibility covered the Morobe and Sepik districts.
[1] In 1935, the New South Wales Department of Education resumed hiring new teachers and McCarthy returned to Australia to take up a position teaching English and history at Petersham Intermediate and Homebush Junior Boys’ High schools.
In 1938, he gave up teaching to work at Qantas Empire Airways as a flight clerk for the company's flying-boat service.
[1] Soon after the outbreak of the Second World War, McCarthy enlisted in the Australian Imperial Force and was posted to 2/17th Battalion.
As a result of his service in New Guinea and Bougainville, he was made a Member of the Order of the British Empire, which was published in the London Gazette in March 1947.
Long's decision was based on an account that McCarthy had written of the retreat to Tobruk in the early stages of the North African campaign, published in The Bulletin.
[2] McCarthy worked on his volume, titled South-West Pacific Area–First Year: Kokoda to Wau, concurrently with his diplomatic career.
[1] In 1963, McCarthy transferred to the Department of External Affairs, becoming the Australian minister to the United Nations for three years.