Unionville's name was borrowed from Ira White's Union Mills built in 1839, which in turn was titled for the Act of Union 1840 of Upper Canada and Lower Canada. Unionville was a "Police Village" within what was then termed Markham Township within York County.

The name "Unionville" continued a valid postal address until the early 1990s at which time most addresses were changed to "Markham". In 2009, Markham Town Council returned the name "Unionville" for that portion of postal addresses within its Ward 3. However, the Post Office used "Unionville" as the postal address for a larger delivery area that included the farms that it considered to be "in Unionville". Unionville's historical edges, therefore, maybe based on the historic Post Office delivery area for the name "Unionville". Since January 1, 1971, Unionville has had no proper municipal boundary.

In the 1960s, major housing advancement came to Unionville and is still ongoing. Having old buildings vacant at low cost, a number of antique stores sprang up and for a while in the 1970s Unionville ranked high on the list of regions to go to get antiques. Over the next decades, the antique places retreated being replaced by high-end antique and replica outlets, restaurants, pubs, and clothing establishments. Tourism was born. Many of the constructions have been spruced up, extended and upgraded to meet this new reality.