[4] The highway proceeds northwest as Communication Road, passing between farmland on the outskirts of Chatham.

The highway progresses from the outskirts of Chatham to downtown before turning onto St. Clair Street and travelling northwest.

[8] Oungah is the only community between Chatham and Wallaceburg, where the highway divides a large swath of farmland established in the fertile soils of the region.

As it enters the town, it turns onto McNaughton Avenue and crosses a CSX railway line then the Sydenham River.

The route turns west onto Dufferin Avenue and proceeds out of the town, crossing the CSX railway a second time.

[3] Seven kilometres (4.3 mi) north of the county line, the highway crosses the W. Darcy McKeough Floodway, a channel constructed in 1984 to protect Wallaceburg from flooding.

[3][11] As the highway approaches the south end of Sarnia, it widens into a divided four-lane expressway and passes through Chemical Valley, the location of several industrial plants.

[12][13] The route enters Aamjiwnaang First Nation at La Salle Line where the surroundings abruptly switch to forest from a mix of industry and farmland.

Approximately three kilometres (1.9 mi) east of the intersection, the highway narrows back to a two lane road.

[1] The designation came just weeks before the 1934 Ontario general election, which saw Lambton West incumbent Conservative Member of the Legislative Assembly Andrew Robinson McMillen replaced by Liberal William Guthrie.

[16][17] In the decade following World War II, automobile use in North America increased dramatically, inundating many highways on the approach to and within urbanized areas with heavy traffic.

As a result, freeways and bypasses were constructed throughout the province, allowing drivers not destined for those locations avoid congestion.

[29] Plans for the parkway were crafted over the next several years before being presented to minister of highways Charles MacNaughton in the spring of 1965.

were built south of Sarnia along the future highway corridor, an area known since the 1940s as Chemical Valley.

[32][33] Following several years of negotiations with the Aamjiwnaang First Nation (then the Chippewas of Sarnia),[34][35] construction of the new inland route of Highway 40 began in July 1972.

[37] A second contract, awarded in 1974, extended construction south an additional 11.0 km (6.8 mi) to Lambton County Road 2 (Bentpath Line) near Sombra.

[38] The third and final contract was awarded in late 1975 to complete the remaining 13.2 km (8.2 mi) south of County Road 2.

[59] 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.

[60] The following table lists the major junctions along Highway 40, as noted by the Ministry of Transportation of Ontario.

[23] The DHO maintained the 8.0 km (5.0 mi) portion from the Sarnia city limits at Indian Road to Highway 402.