Slaton was born in Meriwether Co., GA, graduated from the University of Georgia, and practiced law in Atlanta for 68 years. He served in the Georgia House of Representatives, in the Georgia Senate, and as both acting governor and governor of the state (1911-1912 and 1913-1915). Slaton worked for fiscal conservatism in government and the regulation of public utilities. However, he is probably best known for commuting the sentence of Leo M. Frank from execution to life imprisonment in 1915 after Frank, a Jewish factory manager in Atlanta, was convicted of murdering a white adolescent employee named Mary Phagan. Frank was subsequently lynched. Although that was the end of his political career, in 1928 John M. Slaton was elected president of the Georgia Bar Association.