The route travelled at the southernmost point in Canada, along or near the shoreline of the Detroit River and Lake Erie between Windsor and Leamington, with Highway 3 as the terminus at both ends.

Highway 18 was an 76.0-kilometre (47.2 mi) route that travelled along or near the shoreline of the Detroit River and Lake Erie between Windsor and Leamington in Southwestern Ontario.

[6] Today it is mostly known as Essex County Road 20; it is four-lanes from Windsor to Amherstburg, and generally two lanes for the remainder of its routing.

In addition, portions of the route through Amherstburg, Harrow, Kingsville and Leamington were maintained under Connecting Link agreements.

Ojibway Parkway, a divided four lane limited-access road, ends at the Windsor city limits at Morton Drive.

[10] After passing through the centre of Amherstburg, where Highway 18 was maintained under a Connecting Link agreement between Brunner Avenue and Lowes Side Road, the route follows the shores of the Detroit River once more before curving east inland.

[4][9] It crosses Big Creek twice, with Knapps Island lying in the middle,[10] before encountering the village of Malden Centre.

Highway 18 was maintained under a Connecting Link agreement in Kingsville between Fox Lane and the Chrysler Canada Greenway, a rail trail along a former branch of the Chesapeake and Ohio Railway.

[4] East of Kingsville, the former route of Highway 18 travels parallel to and north of Lake Erie, providing access to lakeside properties.

[3] Prior to then, it entered Leamington at Albuna Townline, travelling along Seacliff Drive West to Erie Street, where it turned north.

[5] The portion of the route from Forest Avenue and Seacliff Drive to Talbot Street was maintained under a Connecting Link agreement.

In anticipation of the opening of the Ambassador Bridge, as well as the nearby Detroit–Windsor tunnel, Windsor and the surrounding townships sought road improvements between Windsor and Maidstone to alleviate traffic along Talbot Road and bypass or separate several level crossings of the Michigan Central Railway (MCR) beginning in early 1929.

[25] Along with the new Highway 18 designation, which was applied in February 1932, the DHO undertook reconstruction of the unimproved portions of the route as a depression-relief project.

Two sweeping curves were also built between Malden Centre and Harrow, and all unpaved sections were graded and gravel surfaced.

[34] The final gap of gravel between Malden Centre and Harrow was paved in 1936, and a new 76 m (250 ft) wooden bridge built over the River Canard along the right-of-way of the Sandwich, Windsor and Amherstburg Electric Railway.

[36] Although this problem was recognised shortly after the highway was established,[37] work to remedy it did not begin until 1958, when construction began on the Amherstburg diversion.

The divided portion began at Sandwich Street and ended at an intersection on the north side of the Turkey Creek bridge.

[51] Planning to remove the dangerous two lane Brunner Mond bridge in Amherstburg, the site of numerous accidents, was underway by the 1970s.

The bridge crossed a Penn Central Railroad track midway between Texas Road and Brunner Avenue, and formed a major hurdle in the proposal to four lane Highway 18 between Amherstburg and LaSalle.

[52][53][54] On March 1, 1976, Minister of Transportation James Snow officially announced that the rebuilt highway would be four lanes wide.

[60] Widening of 4.7 km (2.9 mi) of Highway 18 between Amherstburg and the River Canard bridge began on July 23, 1985, at a ground-breaking ceremony attended by Minister of Transportation Ed Fulton.

[62] As part of a series of budget cuts initiated by premier Mike Harris under his Common Sense Revolution platform in 1995, numerous highways deemed to no longer be of significance to the provincial network were decommissioned and responsibility for the routes transferred to a lower level of government, a process referred to as downloading.

[63] Essex County Road 45 (Union Avenue) was assumed in order to establish a logical eastern terminus for the route at Highway 3 in Ruthven.