[2] Mills' mother died when was six years old and he was raised by his father in Moruya;[2] one of his older sisters was killed in a car crash when he was a child.
He completed his residency at Royal Prince Alfred Hospital (RPAH), also assisting Frank Rundle with his research on thyroid disease.
[2] He subsequently completed further training in England on a Walter and Eliza Hall scholarship and was admitted to Fellowship of the Royal College of Surgeons in 1938.
He established a small field hospital in the early days of the Malayan campaign and after the fall of Singapore became a Japanese prisoner of war.
[2] After the war's end Mills was admitted to Fellowship of the Royal Australasian College of Surgeons in 1947 and began working as an assistant to Hugh Poate at RPAH.