1996-2004: Liberal dominance

The riding was first established in the 1996 redistribution from parts of York North and Markham—Whitchurch—Stouffville, consisting of the part of Vaughan east of Highway 400 and south of Rutherford Road, and the part of Markham west of Highway 404.^[11][12] Both of the ridings Thornhill was originally part of elected Liberal MPs in 1993, though York North's Liberal vote share was about 17% more of the vote than that of Markham—Whitchurch—Stouffville.^[13] The riding's redistributed result had the Liberals at 60%, triple the amount of the Progressive Conservatives (PCs).^[14] In the 1997 election, Liberal candidate Elinor Caplan, who had previously served in the cabinet of former Premier David Peterson,^[15] won with 59% of the vote, more than double the number received by PC candidate Bill Fisch, who came second.^[16] Similarly to the previous election,^[11] the Liberals had nearly swept the province of Ontario, this time winning all but two seats.^[17]

In 1999, Caplan was appointed to Prime Minister Jean Chrétien's cabinet as Minister of Citizenship and Immigration.^[15] In the 2000 election, Caplan would win again, this time with nearly 65% of the vote and by more than four times the amount won by Canadian Alliance candidate Robert Goldin.^[16] Provincially, the Liberals had won another near-sweep of Ontario.^[18] In 2002, Chrétien moved Caplan to Minister of National Revenue.^[15] During the 2003 electoral redistribution, the boundaries of the district did not change.^[12][19]

2004-2011: Shift to the Conservatives

Peter Kent with then-party leader Andrew Scheer in 2018 stating their position of the status of Jerusalem. The Conservatives' support for Israel was partially responsible for the riding's political shift.^[20]

On February 23, 2004, Caplan announced that she would not run in the next federal election. The Liberals then chose Susan Kadis, who was then a Vaughan City Councillor, as their candidate for the next election.^[6][21][22] In the 2004 election, many ridings with large Jewish populations, Thornhill among them, started shifting toward the Conservative Party. The Liberal Party would end up losing 10% of the vote and they were now at 54.6%, about 20% more of the vote than Conservative candidate Josh Cooper.^[6][23] In the 2006 election, both Kadis and the Conservative candidate got slightly less votes than in 2004.^[24]

Shortly after the 2006 election, Kadis was chosen to be the associate critic for Infrastructure and Communities.^[25] In March 2008, she was appointed National Revenue critic by Opposition Leader Stéphane Dion.^[26] In the 2008 election, the Conservative Party chose Peter Kent, an established media personality who had run in Toronto-St. Paul's in 2006,^[24] as their candidate. Kent would end up winning the riding with 49% of the vote compared to Kadis' 39%, possibly due to the fact that Conservative Prime Minister Stephen Harper promised continued support for Israel.^[20]

2011-present: Conservative stronghold

Kent served as the Minister of State of Foreign Affairs for the Americas until a January 2011 cabinet shuffle when he was promoted to Minister of the Environment. This was met with some criticism as he was the fifth Environment Minister in five years. It also signalled Prime Minister Harper's determination to increase support in the Greater Toronto Area.^[27] In the 2011 election, the Conservative Party targeted the riding as part of their strategy to gain a majority government and both Prime Minister Harper and Liberal leader Michael Ignatieff had campaign stops in the riding. Kent ended up increasing his vote share to 61%, compared to 24% for the Liberal candidate.^[28][29]

During the 2012 redistribution, the area of the riding east of Bayview Avenue was divided along Highway 407, with portions being transferred to Richmond Hill and the new riding of Markham—Thornhill.^[19][30] Concerns were raised about dividing the Thornhill neighbourhood into two ridings, suggesting that Concord (which includes Vaughan Metropolitan Centre) be excluded from the riding instead. The name of the riding was also an issue, as the proposed name, "Vaughan—Thornhill" was thought to exclude the Markham portion.^[4] Markham was initially added to the name before Kent suggested reverting it back to "Thornhill". His suggestion was accepted by the commission.^[31] The redistributed result put the Conservatives 2% higher than the actual result.^[29][32] In July 2013, Kent was shuffled out of cabinet, becoming a backbench MP.^[33] During his tenure, critics had described him as "Canada's worst environment minister."^[34][35]

In the 2015 election, Kent lost about 5% of the vote, now receiving 58.6% to the Liberal candidate's 33.7%.^[32] Shortly after the 2015 election, Kent became the Conservative Foreign Affairs critic.^[36] After a 2017 shadow cabinet shuffle, Kent became the Conservatives' Ethics critic.^[37] In 2019, Kent won his fourth consecutive election with 54.6% of the vote to Liberal candidate Gary Gladstone's 35.4%.^[38][39] After the 2019 election, Kent was appointed critic on immigration, refugees, and citizenship.^[40] In 2021, Kent announced he would not run in the next election.^[41]

Following this, Melissa Lantsman won the nomination to represent the Conservative Party in Thornhill. On September 20, 2021, Melissa Lantsman won the riding of Thornhill by a 15-point margin.