Once a municipal village, Thornhill is now a community and postal designation geographically split into two municipalities along Yonge Street, City of Vaughan to the west and the Town of Markham to the east.  With the recent expansion of Thornhill to the west and north, areas are being further blurred and referred to as "Upper" or "North" Thornhill. Although there is a defined postal designation, it is difficult to map the actual boundaries of the village.

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.