Highway 21 was the first King's Highway in
Lambton County when it was assumed in 1927 between Highway 3 at Morpeth and Highway 7 at Reece's Corners. The original section of highway was rebuilt from a muddy trail to a
plank road around 1860. When
James Miller Williams, a
Hamilton businessman, set out one day during a drought to dig a well, he chose a spot downhill from an existing
oil seep in the village of Black Creek. Instead of encountering water, Williams hit a shallow oil deposit. As a result of the ensuing oil boom, which would begin the petroleum industry in North America, Williams laid out the village and changed its name to Oil Springs. Two competing plank road companies were formed, the Black Creek Plank Road Company (of which Williams was a principal investor) and the Sarnia to Florence Plank Road Company, both of which aimed their roads through Oil Springs. Although both roads were constructed, the former company was more prosperous in its endeavours; in 1886, a significant portion of the Sarnia to Florence Plank Road was closed up and turned over to local property owners. The Black Creek Plank Road Company meanwhile had transformed the muddy
quagmire of a path into a well-maintained road. By 1863, three miles of road south of Wyoming had been paved, and the remainder south to Oil Springs planked (the Sarnia Road followed two years later). However, as the oil boom faded, so too did improvement to the road. . A
Bowstring Arch bridge was constructed to replace the existing county-built bridge shortly after the department designated Highway 21. On May25 and June1, 1927, the
Department of Highways assumed the
unpaved road between Highway 7 at Reece's Corner and Highway 3 at Morpeth, via
Dresden,
Thamesville and
Ridgetown as
Provincial Highway 21, which was changed to the current ''King's Highway 21'' in 1930. That year, the department set out to improve the new highway. Concrete slabs were laid between Petrolia and Highway 7, as well as along a section between Thamesville and Dresden. The following year, the route was paved between Dresden and
Edys Mills before the effects of the
Great Depression forced the department to concentrate on paving
Highway 22. The election of a new government in mid-1934 led to the resumption of work in June as a depression relief project. New equipment (namely a Caterpillar Excavator), as well as the expertise of Andy Newman, an engineer who was hired when he demonstrated his abilities with the machinery upon passing a construction site on his drive home. Newman, who helped design the machine that nobody else could operate, allowed work to proceed at a much faster rate than before. The machinery could dig quicker than 50 men, and the effort showed when the gap between Petrolia and Edys Mills and the remaining gaps between Dresden and Thamesville were graded and paved by the end of the summer. On October 19, 1934, Highway 21 was officially opened by
Robert Mellville Smith, deputy minister of the Department of Highways. On April 4, 1934, Highway 21 was assumed through Huron County as far north as Goderich, which was followed by the assumption of a section through
Bosanquet Township on April 18, creating a concurrency with Highway 7 from Reece's Corners to Thedford. From there, the route travelled through Thedford to Port Franks, where it merged into the present highway. A final extension to Owen Sound was assumed on May 15, 1935, bringing the highway to its greatest length of . near
Goderich Meanwhile, on April 11, 1934, the department assumed control of a road connecting Highway 7 with Forest as Highway 21A. By 1938, Highway 21A had been renumbered as Highway 21, and Highway 21 through Thedford renumbered as Highway 82. Beginning in 1960, a small bypass of Highway 21 was constructed on the north side of Goderich, avoiding a nearby
hairpin turn. The curving structure over the Maitland River was completed in mid-1961 at a cost of
C$1.39 million and opened ceremoniously on July 17, 1962. During the early 1980s, the construction of Highway 402 east from Sarnia resulted in a shift in the route of the highway. The route was extended north from Reece's Corners to Exit 25, while the section from Highway 7 north to Exit 34 was
"downloaded", or transferred to the local municipality in which it resided. With Highway 402 as the connecting provincial link between the two segments of Highway 21, the two parclo interchanges each include a directional ramp to facilitate traffic.
Further transfers were performed in 1997 and 1998. On April 1, 1997, the section of Highway 21 from
Highway 401 south to Morpeth was transferred to
Kent County. On January 1, 1998, the section between Highway 401 and Highway 402 was transferred to Kent and Lambton counties. == Major intersections ==