Markham—Unionville is located in the City of Markham within an area bordered by a line commencing at the Highway 404-407 interchange, then east along Highway 407 to McCowan Road, north on McCowan Road to 16th Avenue, east on 16th to Highway 48, then to the northern city limit, then along the northern and western city boundaries to the 404-407 interchange.
It is located in the province of Ontario, and covers suburban areas north of Toronto. It was created in 2003 from Markham. The federal riding has been represented by John McCallum, until he switched to the newly formed Markham—Thornhill riding for the 2015 election. Markham—Unionville was the only seat that the Liberals lost among those they held after the 2011 election, though the riding's boundaries changed considerably and would have been won by the Conservatives in 2011 based on the redistributed results.
Most of (54%) Markham—Unionville (mostly south of Highway 407) was redistributed into the new Markham—Thornhill riding for the 2015 election. The area north of Highway 407 and west of McCowan Road remained in Markham—Unionville. The new Markham—Unionville riding also took in newer subdivisions in the northwest corner of the city, which were previously in the riding of Oak Ridges—Markham. 51% of the new riding came from Oak Ridges—Markham.