He was born in Saint Johns, Canada E. The son of Edward Hartley Dewart, a Methodist minister who was a staunch advocate of prohibition, the younger Dewart inherited his Liberal politics but broke with his father on the temperance issue becoming a firm opponent of 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. In 1919, he became leader of the Ontario Liberal Party. He resigned as leader in 1921 due to opposition within his caucus to his anti-prohibitionist stance.
He returned to private practice and died at his home near Uxbridge in 1924.
He ran unsuccessfully as the Liberal candidate in the 1904 federal election for Toronto South and in the 1911 federal election for York Centre.
