The riding included all of Dufferin County plus the municipalities of Caledon, Erin, West Luther Township, Arthur, Southgate and Mount Forest.
The provincial riding of Dufferin—Peel—Wellington—Grey was created when the Harris government passed a bill reducing the number of ridings electing Members of Provincial Parliament (MPPs) in the Legislative Assembly so that they were the same as the number of federal Members of Parliament from Ontario.
In April 2002, Tilson resigned his seat in order to give the newly elected party leader and Premier of Ontario, Ernie Eves, an opportunity to enter the provincial legislature through a by-election.
Eves won the by-election and retained his seat in the subsequent 2003 election although his government was defeated.
Eves subsequently resigned as party leader and, on January 31, 2005, he resigned his seat in order to give his successor, John Tory the opportunity to enter the legislature through a by-election.