The riding of Markham—Thornhill was vacated on February 1, 2017,[1][2] following the appointment of Liberal MP and Minister of Immigration, Refugees and Citizenship John McCallum as Ambassador to China.

[4] Justin Trudeau's director of appointments and former senior Queen's Park staffer Mary Ng,[5] defeated small business owner Nadeem Quereshi[6] and technology entrepreneur Afraj Gill[6] for the Liberal nomination on March 4, 2017.

[10][11] Radio host and newspaper columnist Gavan Paranchothy defeated CIBC economist Theodore Antony,[6] former senior Queen's Park staffer Lara Coombs,[6] former Don Valley East MP Joe Daniel,[6] and parental rights advocate John Himanen[6] for the Conservative Party nomination on March 8.

Small business owner Gregory Hines defeated insurance broker Marco Coletta for the NDP nomination.

Independent candidate Above Znoneofthe is a resident of Oshawa, formerly named Sheldon Bergson, who legally changed his name in 2015 so that he could register as a "none of the above" protest candidate in the 2015 federal election;[13] although he did not complete the process in time to register for that election, he first ran in the provincial Whitby—Oshawa by-election of 2016.