Thornhill’s pioneers arrived in 1794. It was a settler named Asa Johnson who established on the Vaughan side of Thornhill. The village has always existed on two different municipalities. This separation has not stopped Thornhill from establishing its own character as a community.

The community takes its name from Benjamin Thorne who settled here in 1820. Thorne managed a gristmill, a sawmill, and a tannery in the area. Not surprisingly the community became known as Thorne's Mills, and thereafter Thorne's Hill, and finally Thornhill.

Major thrust for its growth is its location along Yonge Street as it provided access to the City of Toronto to the south. This access was increased with the arrival of the electric street railway along Yonge Street in 1898 and further enhanced in the 1920s, with the arrival of the automobile.

In 1931, Thornhill became a "Police Village". The formation of the Police Village gave Thornhill its own political boundaries. The village was headed by a reeve.

Today, in Old Thornhill along Yonge Street, over one hundred historic buildings stand as note of Thornhill's past. The village recognize its village identity in part thanks to the post office which retained the Thornhill name and serviced homes and businesses within the former village's boundaries.