Markham was first surveyed as a township in 1793 by William Berczy, who in 1794 led 75 German families including the Ramers, Reesors, Wheters, Burkholders, Bunkers, Wicks and Lewis from Upstate New York to an area of Markham now known as German Mills. Each family was granted 200 acres (0.81 km2) of land; however due lack to roads in the region many opted to settle in York (now Toronto) and Niagara. German Mills later became a ghost town. Between 1803 to 1812, another attempt at settling in the region was made. The largest group of settlers were Pennsylvania Dutch, most of whom were Mennonites. These highly skilled craftsmen and knowledgeable farmers were able to settle the region and founded Reesorville, named after the Mennonite settler Joseph Reesor. In 1825, Reesorville was renamed to Markham having taken the name of the unincorporated village (see Markham Village, Ontario). By 1830, a large influx of Irish, Scottish and English families began immigrating to Upper Canada, many settling in Markham. Markham's early years blended the rigours of the frontier with the development of agriculture-based industries. The many rivers and streams in the township soon supported water-powered saw and gristmills and later wooden mills. With improved transportation routes, such as the construction of Yonge Street in the 1800s, along with the growing population, urbanization increased. In 1842 the township population was 5,698; 29,005 acres (117.38 km2) were under cultivation (second highest in the province), and the township had eleven gristmills and twenty-four sawmills. By 1850, the first form of structured municipal government formed in Markham. By 1857, most of the township had been cleared of timber and was under cultivation. Villages like Thornhill, Unionville, and Markham greatly expanded. In 1851 Markham Village "was a considerable village, containing between eight and nine hundred inhabitants, pleasantly situated on the Rouge River. It contains two grist mills ... a woollen factory, oatmeal mill, barley mill, and distillery, foundry, two tanneries, brewery, etc., a temperance hall and four churches... ." In 1871, with a township population of 8,152, the Toronto and Nipissing Railway built the first rail line to Markham Village and Unionville, which is still used today by the GO Transit commuter services.
In 1972, Markham was incorporated as a town, as its population skyrocketed due to urban sprawl from Toronto. In 1976, Markham's population was approximately 56,000. Since that time, the population has more than quintupled with explosive growth in new subdivisions. Much of Markham's farmland has now disappeared, but is still found north of Major Mackenzie Drive. Controversy over the development of the environmentally sensitive Oak Ridges Moraine will likely curb development north of Major Mackenzie Drive.
As of 2006, Markham comprises six major communities, which include Buttonville, Cornell, Markham Village, Milliken, Thornhill, and Unionville. Since the 1980s, the city has been recognized as a suburb of Toronto. Many high-tech companies have head offices located in Markham for the relative abundance of land, low tax rates and good transportation routes. Broadcom Canada, ATI Technologies (now known as AMD Graphics Product Group), IBM Canada, Apple Computer Canada, Motorola Canada, Honeywell Canada and many other well-known companies have chosen Markham as their home in Canada. Hence, the city has been branding itself as Canada's "High-Tech Capital". An Ontario Historical Plaque was erected in front of the Markham Museum by the province to commemorate the founding of Markham's role in Ontario's heritage.
Town council voted on May 29, 2012, to officially change Markham's legal designation from "town" to "city"; according to councillor Alex Chiu, who introduced the motion, the change of designation merely reflects the fact that many people already think of Markham as a city. Some residents objected to the change because it will involve unknown costs without any demonstrated benefits. The designation officially took effect on July 1.