Schreiner was elected as MPP for the riding of Guelph in 2018, making him the party's first member of the Ontario Legislative Assembly.

[7] Subsequently, the party's popularity declined in the 2011 and 2014 elections during tightly contested races between the Progressive Conservatives and ruling Liberals.

[13] Under de Jong's leadership, the party fielded a full slate of 107 candidates in the 2007 provincial election, receiving over 8.0% and nearly 355,000 votes.

[14]De Jong announced his resignation as leader on 16 May 2009, at the Green Party of Ontario Annual General Meeting.

[15] Following his resignation, de Jong was replaced by Toronto entrepreneur Mike Schreiner, who was the sole candidate in the party's leadership race.

[16] The Greens failed to win seats in the subsequent 2011 and 2014 provincial elections, though Schreiner received 19% of the vote in Guelph in 2014.

[25] In the lead-up to the 2022 election, the party released policy papers focused on housing,[26] climate change[27] and mental health.

[28] Its 2022 platform identified three priorities: a caring society, focussed on improving equitable healthcare, education, and social services; connected communities, focussed on tackling housing affordability by building more infill development, strengthening protections for renters and addressing speculation in the housing market; and new climate economy, focused on achieving net-zero emissions by 2045 by replacing fossil fuels with renewable energy sources, growing green jobs and protecting the environment.

[30] Its elected members have also called for more stringent tenants protections[31] and for the province to partner with non-profit and co-operative housing providers to build affordable non-market homes.

"[26] They included building more inclusive neighbourhoods through missing middle and midrise development, protecting farmland and other natural land from urban sprawl, building and maintaining a provincial affordable housing supply, ending chronic homelessness, strengthening protections for renters and addressing speculation in the housing market.

[26] The Toronto Star editorial board endorsed the plan, referring to it as "an ambitious document that proposes tackling the housing crisis from all vantage points.

[29] It has called for an end to the province's offshore wind moratorium in order to increase access to renewable power.

[29] Party leader Mike Schreiner was vocal in opposing the Ford government's plan to allow development on southern Ontario's Greenbelt, which was ultimately reversed in 2023.

GPO policy calls for an end to the publicly funded Catholic school system, a merger that it claimed would save millions of dollars in duplicate administrative costs.

The Green Party of Ontario believes in modernizing the social safety net to account for present-day challenges.

[29] It has been an advocate for a universal Basic Income for all Ontarians, in order to provide economic security while at the same time cutting red tape and bureaucracy.

In its 2022 platform, it pledged to work with the federal government to ensure continued funding for universal access to ten-dollar-a-day care.

They have also proposed road pricing (including tolls, parking levies and land-value taxes near subways) to pay for public transit.