Thornhill is situated along the northern border of Toronto, centred on Yonge, and is also immediately south of the City of Richmond Hill.

Of particular importance was the arrival of Benjamin Thorne (January 4, 1794 – July 2, 1848) in 1820 from Dorset, England,[3] who was operating a gristmill, a sawmill, and a tannery in the community.

Stagecoaches travelled between Holland Landing (Lake Simcoe) and York (Toronto) as Yonge Street's road conditions improved with new stonework.

Thornhill's location along Yonge Street, a major transportation route, proved beneficial to the community's growth throughout much of the twentieth century.

Thornhill's administration reverted to Markham and Vaughan, which were enlarged in territory and upgraded to Town status at this time.

In the summer of 2020, after the emergence of the COVID-19 pandemic, the area around Hefhill Park, near Bathurst and Centre Streets, began experiencing a problem with its coyote population.

[4] As reported by the Toronto Star, Thornhill residents' "daily routines have been completely altered after a pack of coyotes living nearby appears to have lost its fear of humans".

[5] Residents of Thornhill continue to report sightings and attacks by coyotes to their local and regional governments as the issue remains unresolved.

It is home to a significant number of Jewish, Chinese, Korean, Iranian, Indian, and Italian people.

According to 2001 Federal Census data, the electoral district of Thornhill (which is not entirely congruent with the neighbourhood) consists of Chinese, the largest visible minority, accounting for almost 11% of total residents (12,610), followed by South Asian (6,595), Black (2,665), Korean (2,660), Filipino (2,535), and West Asian (2,355).

[6] According to the 2009 Report of Canada's Demographic Task Force, Thornhill-Vaughan was in 2001 home to more than 33,000 members of the Jewish community out of 55,000 in this area.

[8] Located at Bayview and John Street (on the Markham side), the community centre features a double arena (home to the Thornhill Skating Club, Markham Majors and Islanders hockey clubs (with an east rink named for Bib Sherwood in 1999), therapy pool, gym room, running track, multi-purpose rooms and Markham Public Library branch.

Unlike many of York Region's farmers markets that are outdoor, it is housed in a permanent building structure.