The West Riding initially consisted of the Townships of Delaware, Caradoc, Metcalfe, Mosa and Ekfrid, and the Village of Strathroy.

In 1903, it was redefined to consist of the townships of Caradoc, Delaware, Ekfrid, Metcalfe and Mosa, the town of Strathroy, and the villages of Glencoe, Newbury and Wardsville.

In 1914, it was redefined to consist of the townships of Adelaide, Lobo, Delaware, Caradoc, Metcalfe, Mosa, Ekfrid, McGillivray, Williams East and Williams West, the towns of Park Hill and Strathroy, and the villages of Ailsa Craig, Glencoe, Newbury and Wardsville.

In 1924, it was defined as consisting of the part of the county of Middlesex lying west of and including the townships of McGillivray, Williams East, Lobo and Delaware.

In 1947, it was redefined to consist of the county of Middlesex, excluding the townships of North Dorchester, London, West Nissouri and Westminster.