In July 1940, he volunteered for overseas service with the Second Australian Imperial Force, and was subsequently assigned to the 2/25th Infantry Battalion, serving with them in the Middle East.

In 1945, he was posted to 112 Military Hospital in Brisbane, and he was discharged from the 2nd AIF in early 1946, but remained active in the reserves, eventually retiring in 1957 with the rank of lieutenant colonel.

[9] Mayes' vision was "I see a 350 bed hospital as our goal, covering all major specialties, with excellent diagnostic facilities and operating theatres".

The hospital had an annual fundraising Scottish fair at Brisbane City Hall each year for which Mayes cooked haggis.

Pixie Annat, matron and CEO of St Andrew's War Memorial Hospital remembers:[12]"Dr Mayes was a man of great vision and courage, generous of spirit and a Scot to his bootstrap.