He was known as a pioneer of the Australian Dictionary of Biography, and also as the official biographer of Billy Hughes (the eleventh Prime Minister of Australia).
Returning to Australia, in 1934 Fitzhardinge began working at the Commonwealth National Library as the research officer responsible for Australian collections.
From 1944 to 1945, Fitzhardinge was seconded to Canberra University College to teach Australian history to diplomatic cadets; one of his students there was Donald Horne.
In retirement, Fitzhardinge lived on a farm near Queanbeyan, New South Wales, which he and his wife Verity Hewitt (married 1936) had bought in 1959.
Titled William Morris Hughes: A Political Biography, it was published in two volumes: That Fiery Particle, 1862–1914 (1964) and The Little Digger, 1914–1952 (1979).