Born in Toronto, Robert John Fleming was of Irish ancestry, the son of William and Jane (Cauldwell) Fleming. Educated in Toronto public schools he first entered the business world in real estate. From there he moved on to the Toronto Railway Company becoming the general manager in 1905.
Fleming's political career began as an Alderman 1886 to 1890. In 1892 was elected mayor of Toronto. He was reelected in 1896 as mayor. From 1898 to 1904 he as an assessment commissioner of Toronto. In 1894 presided over the National Exhibition Conventio at Montreal. Fleming was president in 1895 of the Dominion Prohibitory Alliance. He strongly believed in the prohibition of liquor traffic.
Fleming ran for mayor again in 1923 against Mayor Charles A. Maguire.
As a businessman he was general manager of the Toronto & Niagara Power Company, general manager of the Toronto Electric light Company and general manager of the Electrical Development Company . He as also director of the Toronto Board of Trade. He was a member of the Toronto Harbor Commission in 1921.
Robert John Fleming died in 1925 and was buried in Mount Pleasant Cemetery, Toronto.