McDonald was born in Albany, Western Australia, to Mary Jane (née Elder) and Angus McDonald. He boarded at Scotch College, Perth, and then studied law by correspondence at the University of Adelaide. He was called to bar in 1910. McDonald enlisted as a private in the Australian Imperial Force in 1916, and later served as a lower-level officer with various artillery units in France and Belgium. He had reached the rank of lieutenant by the end of the war. On his return to Australia in 1919, McDonald joined the law firm of Robert Thomson Robinson (a fellow Albany native), eventually becoming a partner. He was a part-time lecturer at the University of Western Australia from 1928 to 1931, and also served as the foundation vice-president of the Australian branch of the International Commission of Jurists.Robert Ross McDonald – Biographical Register of Members of the Parliament of Western Australia. Retrieved 16 May 2016.
