Member of Provincial Parliament

Contesting Davenport for the provincial NDP in the 2018 Ontario election, Stiles ran against Liberal incumbent Cristina Martins, who faced criticism for her government's handling of the Davenport Diamond rail overpass. Stiles campaigned for the electrification of the nearby GO Transit line and ensuring all “amenities” associated with the Davenport Diamond rail bridge are completed.^[9][10] She defeated Martins, becoming the riding's second NDP MPP.^[10][11]

Stiles was the party's education critic from 2018 to 2022.^[12]

NDP leadership

Following the 2022 provincial election, the NDP remained in opposition and party leader Andrea Horwath stepped down. MPP Peter Tabuns became interim leader until the party could hold a leadership election in 2023. Stiles announced that she would run for NDP leader on September 22, 2022.^[13] During her campaign, she received endorsements from eight fellow MPPs.^[3]

During her campaign, Stiles outlined five priorities for the NDP and the province.^[4] She ran on climate action by creating jobs in sustainable industries and investing in green infrastructure.^[14] She called for "true reconciliation" by protecting Indigenous rights and addressing issues such as clean drinking water, treatment in the justice system and shelter.^[14] Labour was another one of her priorities, and she has stated that she would like to focus on creating jobs where workers' rights and safety are respected.^[14] Another commitment was reforming the province's electoral system and moving away from first-past-the-post.^[14] Stiles has also criticized what she perceives as the Conservative's efforts to increase privatization of government services, and committed to improving public education, healthcare, and social security.^[14]

The NDP announced on December 5, 2022, that Stiles was the only candidate in the leadership election, and hence leader-in-waiting.^[15][16] While several other MPPs had explored running for leader, none were able to meet the entry requirements—which included raising a $55,000 fee—before the December 5 deadline.^[3] She was scheduled to be acclaimed as leader on March 4, 2023, but the party's provincial council voted to move the date up to February 4.^[15][17][18] Premier Doug Ford congratulated Stiles in a brief statement posted to Twitter stating "I'm looking forward to many spirited debates as we both strive to make Ontario the best place to live, work and raise a family."^[19]

Stiles was formally confirmed on February 4, 2023, taking office as leader of the NDP and becoming the leader of the Official Opposition in Ontario.^[20][21]

In October 2023, Hamilton Centre MPP Sarah Jama was removed from the NDP caucus for failing to abide by the terms of an agreement between herself and Stiles by taking a "number of unilateral actions" without party endorsement, including statements regarding the 2023 Israel-Hamas war, being uncooperative with NDP colleagues, and threatening Premier Doug Ford with legal action without first consulting with her caucus.^[22][23][24] Stiles had originally defended Jama, and had met privately with Jama beforehand, asking her to remove her statement and apologize; Jama subsequently apologized for her posts, but refused to remove the statement--in defiance of party direction--and instead pinned her statement to the top of her feed on X.^{[25][26][27][28]} Jama said: "I don't plan on apologizing..."^[29] As a result of her removal of Jama, the NDP was divided over the issue;^[30] while concerns were expressed in the NDP that Jama had not been ejected sooner for refusing to remove her statement, with some in the NDP were critical of the political damage that resulted from the delay in expelling Jama (with one MPP having to be convinced not to quit the NDP over the delay), at the same time Stiles received criticism from some for her decision and faced calls to resign from the leadership by two constituency associations.^[31]