The district consisting of part of the Regional Municipality of York, and in particular, the city of Markham as follows: commencing at the intersection of the southerly limit of the city with Ontario Highway 404; thence northerly along said highway to 16th Avenue; thence easterly along said avenue to McCowan Road; thence southerly along said road to Highway No. 7; thence easterly along said highway to 9th Line; thence southerly along said line to the southerly limit of said town; thence westerly along said limit 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, for its entire history, has been represented by John McCallum.
Most of (54%) Markham—Unionville (mostly south of Highway 407) will be redistributed into the new riding of Markham—Thornhill 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 takes in newer subdivisions in the northwest corner of the city, which are currently in the riding of Oak Ridges—Markham. 51% of the new riding comes from Oak Ridges—Markham.