Markham-Unionville is part of the Town of Markham, in the Regional Municipality of York.
Its boundaries are officially described as: commencing at the intersection of Highway No. 404 with Highway No. 407; thence easterly along Highway No. 407 to McCowan Road; thence northerly along said road to 16th Avenue; thence easterly along said avenue to Highway No. 48; thence northerly along said highway to the northerly limit of said town; thence westerly along said limit to Highway No. 404; thence southerly along said highway to the point of commencement.
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 will remain 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.