[1] Following her graduation from Dalhousie University, May worked as an environmental lawyer in Halifax before moving to Ottawa in 1985, joining the Public Interest Advocacy Centre as the associate general counsel.

As senior policy advisor, May was deeply involved in the negotiation of the Montreal Protocol, an international treaty designed to protect the ozone layer.

She resigned on principle from the position in 1988 over permits for construction of a dam granted without environmental assessments, which were later determined to be illegal by a federal court.

In 2006, after building the Sierra Club in rural Nova Scotia into a nationally effective organization in Halifax, May resigned to run for leadership of the Green Party of Canada, winning on the first ballot with 66% of the vote.

[16] May first became known in the Canadian media in the mid-1970s through her leadership as a volunteer in the grassroots movement against proposed aerial insecticide spraying on forests near her home on Cape Breton Island, Nova Scotia.

In the course of the litigation, her family sacrificed their home and seventy acres of land in an adverse court ruling to Scott Paper.

[24][25] In April 2006, May announced her resignation as the Sierra Club's executive director in order to seek the leadership of the Green Party of Canada.

As one of her last major acts she participated in a poll of experts that determined that Progressive Conservative Brian Mulroney was Canada's "greenest" Prime Minister for an award presented by Corporate Knights magazine.

[29][30] In April 2007, during a speech by May to a London, Ontario United Church of Canada, she condemned Prime Minister Stephen Harper's stance on climate change, comparing it to "a grievance worse than Neville Chamberlain's appeasement of the Nazis.

In 2010, following a survey of potentially favourable electoral districts across the country, May announced her intention to run in Saanich—Gulf Islands, in British Columbia against Conservative cabinet minister Gary Lunn.

[48] Bill C-442 was the first piece of Green Party legislation enacted in the history of Canada, and was passed with unanimous consent by both houses of Parliament.

[50] In December 2014, May presented a petition to the House of Commons by members of 9/11 Truth organizations asking the government to review the September 11 attacks in New York.

[51] In 2012, the NDP Foreign Affairs Critic Paul Dewar declined to present a similar petition by another 9/11 Truth group to parliament.

[59] Prior to the amendment, the bill outlined that those at fault in a spill would only be liable for "costs and expenses reasonably incurred by Her Majesty in right of Canada or a province or any other person".

The original line in the bill said that the National Energy Board "may" recover funds to compensate those affected by a spill, the Green Party amendment changed the "may" to "shall".

[59] In October 2015, Prime Minister-designate Justin Trudeau invited May to be part of the Canadian delegation to the 2015 United Nations Climate Change Conference to be held in Paris, France, in late November 2015; the summit was intended to negotiate post-2020 targets for reducing greenhouse gas emissions and resulted in the GHGPPA.

They were accused of violating a court order requiring those demonstrating to stay five meters back from company work sites, when they allegedly blocked the roadway.

[61] On April 9, 2018, Justice Kenneth Affleck of the British Columbia Supreme Court recommended that May and the others arrested should be charged with criminal contempt in relation to the alleged incident.

[65][66] On May 14, 2018, the special prosecutor handling May's case told Justice Affleck that the province was pursuing a criminal contempt of court prosecution against May.

[72][73] Running on a joint ticket with human rights activist Jonathan Pedneault and proposing that the party adopt a co-leadership model, May won the leadership election on November 19, 2022.

[75] The proposed constitutional change was not approved due to internal disagreement and on July 9, 2024, Jonathan Pedneault resigned as deputy leader, citing personal reasons.

May said she regretted the decision but confirmed that she planned to continue as leader into the 2025 Canadian federal election, and run for re-election in her Vancouver Island riding.

During a visit in 2006 to the Mount St. Joseph's Convent in London, Ontario, May responded to a nun's question about her position on abortion, saying "I don't think a woman has a frivolous right to choose.

"[79] Following initial reports of May's statements, which did not include the full quote, prominent Canadian feminist Judy Rebick announced that she was withdrawing her previous support of May and the Green Party because of May's questioning "the most important victory of the women's movement of my generation".

[79] In a 2011 interview with the Georgia Straight, May said her position had been "massively misreported" and explained "If a woman is in a situation where she’d like to keep her child and needs support, we also want to be there to support that choice and also to ensure that as much as possible we, in our society, provide—not just for women, but for male partners—responsibility, birth-control information in order to avoid unwanted pregnancies … So, it’s a mixed and nuanced position, but there’s absolutely no wiggle room on maintaining the right of women in this country to safe and legal abortions."

[81] May's reasoning was that despite her own personal pro-choice views, Green Party rules do not give her the power as leader to whip votes in caucus.

[81] This position put the Greens in stark contrast to the Liberals and the NDP, both of which require attestations from MPs that they will consistently support the party's stated pro-choice platform.

[87] In October 2014, May sent out a series of tweets defending Jian Ghomeshi, who faced allegations from three women that the radio host was physically violent to them without their consent during sexual encounters.

[93] When asked if she should resign, she said that "a lot of people have given bad press gallery speeches and have gone on to be Prime Minister or gone on to lead other aspects of their lives, time will tell.

[96] CBC's Michael Enright noted that the Press Gallery Dinner has long been home to rowdy behaviour by both politicians and journalists, usually attracting little coverage.