Born in Amherst, Nova Scotia, in World War I Ralston commanded a batallion and was awarded for bravery. He was Minister of National Defence from 1926 to 1930, and was reappointed in that position in 1940 after serving as Minister of Finance from 1939 to 1940.
He supported conscription during World War II, and in 1942 offered to resign when Prime Minister William Lyon Mackenzie King's government would not introduce it. The resignation issue was dropped, but after visiting Canadian troops in Europe in 1944, he argued again that conscription was necessary, and a schism developed in King's cabinet (see Conscription Crisis of 1944). King still had Ralston's letter of resignation from 1942 and forced him to resign, replacing him with Andrew McNaughton.
Ralston died in Montreal in 1948.