He was born in St. Johns, Canada East. The son of Edward Hartley Dewart, a Methodist minister who was a staunch advocate of prohibition, the younger Dewart would staunchly oppose prohibition, a move that would cost him dearly later in life. He studied at the University of Toronto and Osgoode Hall and was called to the Ontario bar in 1887. Dewart set up practice in Toronto and served as crown attorney for York County from 1891 to 1904. He was first elected to the provincial legislature in a 1916 by-election after the death of James Joseph Foy and, in 1919, he became leader of the Ontario Liberal Party. He resigned as leader in 1921 due to opposition within his caucus to his prohibitionist stance. He returned to private practice and died at his home near Uxbridge in 1924.