Educated in Toronto public schools he first entered the business world in real estate.

Fleming's political career began as an Alderman for the Ward of St. David's 1886 to 1890.

He was defeated in 1894 and 1895 but was returned to office in the 1896 Toronto municipal election and again the next year.

He resigned in 1897 to accept appointment as an assessment commissioner of Toronto and held that position until 1904.

Robert John Fleming died in 1925 and was buried in Mount Pleasant Cemetery, Toronto.