The growth and development of Thornhill is directly related to several geographical factors, namely, the development of Yonge Street as an important transportation route, the Don River system running through the village, and lastly, Thornhill's proximity to Toronto.

Thornhill is divided in half between the Town of Markham and the City of Vaughan, and runs along both the east and the west sides of Yonge Street. The first Lieutenant-Governor of Upper Canada, John Graves Simcoe, first developed Yonge Street as a military road.  His initial attempt at trying to find a north- bound route from Fort York (Toronto) along the Carrying Place Trail was considered a failure.

The Carrying Place Trail was an aboriginal route to Georgian Bay along the Humber River system.  Simcoe explored this route in 1792, but found it very difficult and long to travel. On the way back from this trip, a guide showed him a less known aboriginal route.  The trail connected Lake Ontario to Lake Simcoe from York (Toronto).  A year later Simcoe instructed Augustus Jones to survey the trail system that was to be named Yonge Street. (Yonge Street was named after Simcoe's friend and Minister of War, Sir George Yonge.)  By 1793, William Berczy, had cleared the trail as far as the present site of Thornhill.  Later that same year a group of soldiers, the Queen's Rangers, were dispatched by Simcoe to finish the road to Holland Landing (Lake Simcoe).  Yonge Street, the longest road in Canada, was finally completed in January 1794.

In 1792, Simcoe announced a plan to attract settlers to Upper Canada (Ontario).  The plan offered 200 acres of land to pioneer settlers, provided they undertake certain duties in return.  Settlers had to clear and fence 10 acres of grant land, erect a dwelling, and clear 33 feet of land across the front of the property for a road.  This work was to be completed within two years of settlement.  By 1800, all the lots between what is now Steeles Avenue and Langstaff Road were granted to prospective settlers. Simcoe's policies would populate and develop communities throughout Upper Canada.

In the early 19^th century, water was the main source of power that drove industrial machinery.  Thus the Don River played an important role in the early development of Thornhill.  It provided power for saw and gristmills (flourmills) that were established in the area by the new settlers. These mills helped produce lumber to build homes and flour to help produce staple foods such as bread and other baked goods. 

The earliest settlers were either United Empire Loyalists or Americans taking advantage of the generous terms of Simcoe's settlement offer.  In 1801, Jeremiah Atkinson built the first major saw mill on the Don, west of Yonge Street in Thornhill.  A gristmill was constructed in 1802 and gradually, as a result of the mill, the first signs of urban settlement began to emerge.

The years following the War of 1812 saw another wave of immigration take place.  The end of the Napoleonic Wars was characterized by significant social and economic change in Great Britain. The result was a period of emigration of upper class families, newly impoverished by the upheaval, and of servicemen seeking to start a new life.

Of particular importance was the arrival of Benjamin Thorne in 1820.  Thorne set up a warehouse in York dealing in the export of grain and import of iron. When William Purdy's Mill burnt down, Thorne purchased the remains and erected a larger gristmill.  By 1830, Thorne was operating a gristmill, a sawmill, and a tannery.  The small settlement came to be known as Thorne's Mills and then Thorne's Hill after Benjamin Thorne.

In 1828, Thorne and his brother-in-law, William Parson, petitioned the government for a post office.  It was granted in 1829 and the village was officially called Thornhill, with Mr. Parson being its first postmaster.  Thorne became the major influence in the economic life of the village.

A variety of industries, services and artisans had located in Thornhill by the year 1830.  Included among them were two sawmills, a distillery, several blacksmiths and harness makers, two inns, a millwright, a stonemason, a tanner, a weaver, a wheelwright, and a shopkeeper.  (A first account look at Thornhill during this period can be found in the diary recordings of Mary Gapper O'Brien, published as "The Journals of Mary O'Brien".)

Between the years 1830 and 1848, Thornhill experienced a period of sustained growth and prosperity.  The business district of Thornhill developed on Yonge Street in an area between Centre Street and John Street.  Stagecoaches traveled between Holland Landing (Lake Simcoe) and York (Toronto) as Yonge Street's road conditions improved with new grading and stonework.  During this prosperous period, many of the old churches, which survive today, were constructed.  Included among these were Trinity Church (now Holy Trinity), built in 1830 and moved to Brooke Street in 1950; the British Methodist Church on Yonge Street, which was built in 1838 and moved to Centre Street in 1852 was partially destroyed by fire in 1983.

Agriculture prospered during this period as local farmers took advantage of the new mechanical advances, such as reapers and threshers.  In addition, the millers found a ready market for their products in the protected British market.  The village came to acquire further services and the original Crown lots were subdivided to provide for the needs of the new urban class. By 1848, Thornhill was the largest community on Yonge Street north of Toronto, having a population of approximately 700 people.

Thornhill had grown into a bustling, milling centre by the mid-1840s.  However, the factors that fostered its growth, namely government policy, economics, and technology, all evolved and changed around mid-century resulting in an extended period of stagnation.  Foremost of these changes was the British Government's repeal of the Corn Laws in 1846, which ended lower import tariffs for Canadian grain into the British markets.  Farmers and millers were left with a glut of surplus grain.  So serious was the oversupply that Benjamin Thorne was left with large amounts of wheat with no market.  As a result, he went bankrupt.  In 1848, the distressed Mr. Thorne committed suicide soon after selling his asset and satisfying his creditors.  This was the first of a long series of events that eroded the economic base of the village.

The decline in milling continued into the latter part of the 19^th century as less lumber was required for construction and was available for milling.  Agriculture was also in a state of flux by the mid-1870s.  Farmers to protect themselves against fluctuating grain prices, began to engage in mixed farming, much to the disadvantage of the flour millers whose services were required less and less.  This economic downturn was further exacerbated by the decline of soil fertility, which contributed to reduced grain yields.  Floods destroyed many of the remaining sawmills and fire took its toll of the gristmills.  By 1885, most mills had disappeared or had been replaced by steam-powered operations.

In 1896, the new mode of transportation, the Metropolitan Radial Railway (bus-like cabins on rails) reached Thornhill, bringing commuters to and from Toronto. 

 Prior to that time, the only public transit  to the city was a three hour ride by stage coach. 

The electric street railway was a significant improvement in both speed and convenience and for the first time, it was possible to live in Thornhill and work in Toronto.  By the late 1920s, the automobile became a

 popular source of transportation for many people, further facilitating travel on Yonge Street.

Growth, however, remained slow until after World War I, when several subdivisions were registered in the area and Thornhill acquired its three golf courses: Uplands, Thornhill and Toronto Ladies.  Much of the subdivision activity in this period was speculative in nature and not developed until after World War II.  

In 1931, Thornhill became a Police Village. Until that time, Thornhill had been a postal area with no independent municipal status.   Thornhill had been split between the then Townships of Markham and Vaughan along Yonge Street since the initiation of municipal government in 1850. Each Township administering their half of the village.  The creation of the Police Village gave Thornhill its own political  boundaries.  Three elected trustees administered the village at this time.

The full effect of commuters and the northward growth of Toronto were not felt in Thornhill until the years after World War II.   Existing subdivisions were completed and new ones registered as post-war prosperity and the automobile brought families into the suburbs. 

On January 1^st, 1971, the Regional Municipality of York Act came into effect, adopting the Metropolitan system of government. With the creation of a regional government administration, the Police Village of Thornhill ceased to exist and the administration of the community reverted back to the newly created Towns of Markham and Vaughan.

Today, Thornhill is a large urban community with over 49 thousands residents.  Its ethnic composition is very diverse with a large Jewish, Eastern European and Italian population.  It is a community that has grown expansively from its early beginnings, reaching north to Richmond Hill and south to Toronto. Its residents enjoy all modern amenities for shopping, recreational activities, schools, libraries and other conveniences.