By announcement of Gov. Simcoe in 1792, Upper Canada was branched into 19 counties, one being Middlesex County. In 1798 an Act was passed better dividing the Province into 8 Districts, 23 Counties, and 158 Townships; one of which was London Township.

Middlesex County, titled for Middlesex, England, was vested in 1800. Establishment started along the Thames River with the first settlement in what later became Delaware Township.

In 1827, and even later, the County enlarged from Lake Erie to Lake Huron, and from the line of Zone Township to the line of Burford, a tract now wrapping the counties of Middlesex, Elgin, Oxford, Huron, Perth, and Bruce.

Until 1852 the area that is now Elgin County was part of Middlesex County. In 1865 the townships Biddulph & McGillivray were removed from Huron County and added to Middlesex County.