The town of Ajax, Ontario in Canada evolved out of the Defence Industries Limited Pickering Works munitions plant built during the World War II, but its history begins much earlier.

The indigenous peoples were active in the watersheds of the Duffins Creek and the Carruthers Creek since the Archaic period (7000–1000 BCE), although they did not build any major settlements in the area. After the British conquest of New France in 1760, the area became part of the Pickering Township. The conversion of a local trail into the Kingston Road in 1799 contributed to increased settlement in area. In the first half of the 19th century, the Pickering Village, now a neighbourhood in Ajax, evolved as the major population centre of the Township, supported by a timber and agricultural boom. In the mid-19th century, Audley, a smaller community, emerged as a stopover on the route to the port of Whitby. By the 20th century, much of the area of present-day Ajax was farmland, and in the 1920s, the Pickering Beach developed as a cottage community.

After the start of the World War II in 1939, the Government of Canada expropriated most of the farmland in what is now southern part of Ajax, to establish a munitions plant. Operated by Defence Industries Limited (DIL), the government-owned plant employed workers from different parts of Canada. The plant site, along with the residences and the facilities established for the workers, evolved into a self-contained community which was named after the British warship HMS Ajax.

After the plant shut down in 1945, its site was used as a war surplus warehouse and sales outlet, a University of Toronto campus (1946–1949), and a holding camp for war refugees from Europe (1949–1953). The government mandated the Canada Mortgage and Housing Corporation (CMHC) to develop the site and its surrounding area into a modern industrial town. In 1950, Ajax was incorporated as an "improvement district", a form of local administration managed by the Lieutenant Governor's appointees. In 1954, it achieved full municipal status as a town, and an elected council took over the administration on 1 January 1955.

Indigenous peoples

Multiple archaeological sites identified with indigenous peoples are located in the watersheds of the Duffins Creek and the Carruthers Creek, in Ajax and surrounding areas. Projectile points from the Late Paleo-Indian period (10000–7000 BCE) have been discovered in the Duffins Creek watershed.

Scattered chert (flint) tools and flakes dated to Archaic period (7000–1000 BCE) have been discovered in both the watersheds. All these sites are located inland, but it is likely that the indigenous people were also active at the mouths of the streams. The areas where the streams met Lake Ontario at the end of the Last Glacial Period are now under water, which may explain the lack of any archaeological discoveries at the mouths of these streams.

Both watersheds also contain several sites from the early and middle Woodland period (1000 BCE-700 CE). Not much research has been done on these sites, but they appear to be short-term campsites.

In the mid-17th century, the Iroquois people displaced the Petun and the Wendat (Huron) peoples from what is now southern Ontario. The Iroquois Seneca people established river-side villages in nearby areas, such as Ganatsekwyagon on Rouge and Teiaiagon on Humber. However, the watersheds of the Duffins Creek and the Carruthers Creek in Ajax do not show evidence of similar settlements. This may be because of the poor navigability of these streams: Duffins Creek, the larger of the two streams, was navigable by canoes only until 6 km upstream. The presence of sand bar at its mouth prevented boats from entering it, further reducing its suitability for establishing a settlement. Nevertheless, it is likely that the indigenous people used the mouths of these creek for hunting and fishing.

Little is known about the indigenous peoples within the boundaries of present-day Ajax at the time of the first European settlements in the area. At least one Ojibwe family lived at the east edge of the Duffins Creek marshes until the mid-19th century.

Early European settlers

In 1760, French Sulpician missionaries from Ganatsekwyagon reached Duffins Creek, but did not settle there. They were also aware of the existence of the smaller Carruthers Creek, although early French maps (such as Louis Jolliet's 1673 map) do not show this stream.

Like the rest of the Greater Toronto Area, the present-day Ajax was originally a part of the Montreal District of New France, and came under British control in 1760. After being formally ceded to the British in 1763, it became a part of the Province of Quebec. In 1788, it became a part of the Nassau District, which in 1791, became the Home District of Upper Canada. The area was a part of the Edinburgh Township of York County within the Home District. In 1792, the Edinburgh Township was renamed to Pickering Township.

Early European settlement in what is now Ajax began along a trail that later became the Kingston Road. Mike Duffin, an Irish fur trader, is the earliest known European to have settled in the area. In the 1770s, he arrived in what later became Pickering Village, and lived in a cabin located on the east side of the Creek, north of Kingston Road. A bachelor, he cleared a few acres of forest land, but survived mainly by fishing, hunting, and eating tubers. He was murdered around 1791, possibly by indigenous people or one of the travelers for whom he provided lodging. Augustus Jones, who surveyed the area for the Government of Upper Canada in 1791, named the Duffins Creek after him.

The first person to receive a land patent in the region was Major John Smith of 5th Regiment of Foot, stationed at Detroit. This grant of 5,000 acres of land included the part of present-day Ajax to the south of the 3rd Concession Line (later Rossland Road and Concession Road 3). His son David W. Smith, who was stationed at Detroit and Niagara, inherited this land from his father, and received another 1,200 acres of land.

Compared to other parts of the Greater Toronto Area, the Pickering Township developed slowly because much of the land was held by absentee landlords such as the Smith family, or held as Crown and Clergy reserves. The growth accelerated after Lieutenant Governor John Graves Simcoe proposed building a road linking London in the west to Kingston in the east. The government declared the major trail in the area as an extension of Toronto's Dundas Street, and in 1799, American contractor Asa Danforth Jr. converted the trail into Kingston Road. The new road greatly contributed to increased settlement in the present-day Ajax area.

In 1798, William Peak migrated from Hope Township, and settled in the Simcoe Point area, near the mouth of Duffins Creek. A Loyalist from the United States, he had received 200 acres of land from Lieutenant-Governor Simcoe. He traded goods with the natives, and also cleared a small plot of land for cultivation. He built a cabin in 1798 or 1799, and grew corn and potatoes.

A bigger settlement developed around the intersection of Duffins Creek and Kingston Road, gradually evolving into the Pickering Village. In 1807, American Quaker leader Timothy Rogers migrated to the Duffins Creek area with 20 other Quaker families, and settled around the intersection of Kingston Road and Mill Street. By 1809, the population of Pickering Township was 180 people, most of whom lived along the Duffins Creek. The area was still densely forested, and the settlers frequently encountered wild animals including bears.

In 1808, James Powell migrated from New Brunswick and settled in the Carruthers Creek watershed in the eastern part of present-day Ajax. Another prominent early settler was George Washington Post, who purchased a lot in May 1812, and built the Post Inn in 1814.

Timber and agricultural boom

In 1810, Timothy Rogers built saw and grist mills on the banks of the Duffins Creek, around 5 km north of Lake Ontario. The War of 1812 increased military traffic on the Kingston Road, resulting in a better-maintained road, which in turn increased passenger traffic and benefited the local businesses, particularly the inn keepers.

In 1820 Alexander Wood of Toronto built the Elmdale grist mill at the north-east corner of Church Street and Highway 401, where the Church of Jesus Christ of Latter-day Saints was later built. The lumber from the local mills supported the shipbuilding industry at the mouth of the Rouge River.

As the area's population increased, several other businesses sprang up. In 1814 the Post family built the Post Inn (also called Post Tavern) at what is now 365 Kingston Road East, near the intersection of Carruthers Creek and Kingston Road. The inn served as a home for the family, and also operated as a stage station, providing service for stagecoach drivers, passengers, and horses traveling along the Kingston Road. It shut down after the Grand Trunk Railway reached Pickering in 1856, rendering its services obsolete. Other taverns, such as those of Noadiah Woodruff and Samuel Munger, existed nearby in what is now the city of Pickering.

After 1830 an economic expansion of the timber industry in southern Ontario led to the establishment of several sawmills in the area. Subsequently, many immigrants from Ireland, Scotland, and England arrived in the area.

In the mid-19th century the village of Audley developed around what is now the intersection of Audley Road and Taunton Road in northern Ajax. Originally known as Brown's Corners after the owner of a local hotel, it gained importance as a stopover for farmers traveling to the port of Whitby. Besides the hotel and the farms, the village had several other small businesses, a school, a church, and a post office.

By 1861 more than 60% of the original forest in the Pickering Township had been logged, most of it in the southern part. The area south of 5th Concession Line (now Concession Road 5), along the Duffins Creek, had 12 saw mills, 6 grist mills, and one tannery. A smaller number of industries also existed along the Carruthers Creek; the largest of these was along the 3rd Concession Line near the Audley village.

The new residents brought large portions of land under agriculture. The farming was diversified in its initial years, but subsequently became grain-dominated, with wheat, oats, and barley as the major crops. This shift was driven by demand for wheat from Toronto, the demand for wheat from Britain during the Crimean War (1854–1856), and the demand for barley and wheat from the United States.

In 1875 John L. Spink established Pickering Township's largest and most successful grist mill, at what is now the intersection of Kingston Road and Notion Road in Ajax, the site of Moodie's Motor Inn motel. The mill changed owner multiple times, before being destroyed by a fire in 1934.

In the later decades of the 19th century, a fall in the demand for wheat led to economic decline in what is now Ajax, and the Pickering Township in general. The local industry promoted modernization of agriculture and a return to diversified farming involving both grain and non-grain crops. However, the economic decline continued, and the Pickering Township lost over 40% of its population in the second half of the 19th century. An economic depression in the last decade of the century resulted in the closure of several mills and small businesses. The village of Audley was especially affected by the depression.

Early 20th century