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.