Chowns became involved in politics in 2015.^[9][11] In 2017, she was elected as a councillor on Herefordshire Council and became leader of the Green group.^[12][13] In the 2019 Herefordshire Council elections, Chowns was re-elected to the council with 78.6% of the vote.^[14] In 2023, while a councillor, she successfully proposed a motion requiring the council to consider the climate and environmental impact of all future decisions.^[15]

Chowns was elected as an MEP in the 2019 European elections, winning 10.66% of the vote share. In the same election, the Green Party won seven MEPs, up from three.^[16] In the Ninth European Parliament, she sat with the Greens–European Free Alliance and was a member of the International Trade Committee and the delegations to Cariforum and ASEAN.^[17] Chowns focused on climate action, biodiversity, sustainable trade, human rights and European cooperation,^[18] arguing that Europe had a "special responsibility" to lead on achieving zero carbon emissions and tackling global inequality.^[19]

She is an advocate of the re-accession of the United Kingdom to the European Union.^[20] On the tenth anniversary of the Brexit referendum, Chowns called for a roadmap towards UK rejoining the EU, citing concerns over Brexit's economic impact, inequality and the need for closer European cooperation.^[21]

Whilst an MEP, Chowns was arrested on 14 October 2019 in Trafalgar Square defending the rights of Extinction Rebellion protesters to continue.^[22] She was released pending investigation, and was later one of the claimants to challenge the legality of the section 14 order under which she was arrested. On 6 November 2019, the High Court ruled^[23] this blanket use of section 14 by the Metropolitan Police was unlawful.^[24] Chowns declined to sue the police for unlawful arrest, but declared the ruling to be a "victory for the right to peaceful assembly and protest, two cornerstones of our democracy".^[25]

On 4 September 2025, Chowns was elected by Green MPs to lead the Greens in Westminster.^[35] Since 4 July 2024, she has served as the party's foreign affairs and defence spokesperson. On 1 June 2026, she became the party's Commons spokesperson for constitutional affairs and HM Treasury matters. She was previously spokesperson for business and trade, education, housing, communities and local government and social care.^[36]

She has served on the Environmental Audit Committee and on the public bill committees for the Children's Wellbeing and Schools Bill, the Planning and Infrastructure Bill and the Representation of the People Bill.^[36] As of June 2026, Chowns was an officer of the APPGs on Water Pollution (chair), Europe (co-vice chair), House of Lords Reform (vice-chair), Fair Elections, Farming and Human Rights.^[37][38] She supported the Terminally Ill Adults (End of Life) Bill.^[39]

Since being elected, Chowns has secured adjournment debates on the Warm Homes Plan in November 2025^[40][41] and embodied carbon in buildings in July 2026.^[42] She has also introduced four private member's bills: the Water (Agricultural Pollution) Bill,^[43] the Carbon Emissions from Buildings (Net Zero) Bill,^[44] the Building Regulations (Swift Bricks) Bill,^[45] and the Armed Conflict (Requirements) Bill,^[46] addressing agricultural water pollution, carbon emissions from new buildings, biodiversity measures in housing developments, and parliamentary approval for the deployment of UK armed forces.^[38]

Chowns has called for a Water Protection Zone for the River Wye catchment,^[47] presenting constituents' petitions to that effect in September 2024 and March 2026,^[48][49] though ministers have said only that the Environment Agency is considering the option.^[50] She has also linked local river pollution, which government figures attribute largely to agriculture,^[51] to a long-running nutrient neutrality "Lugg moratorium" on housebuilding in her constituency.^[52][53]

Chowns argued on Question Time in November 2024 that the UK should achieve net zero through incentives and by making low-carbon choices easier for people.^[54] She argued in Politics Home that achieving net zero would bring economic and energy security benefits, calling for greater investment in renewable energy and climate action,^[55] and called on the Office for Budget Responsibility to place greater emphasis on meeting net zero targets in September 2024.^[56] In July 2026, Chowns backed proposals to introduce maximum workplace temperatures.^[57]

She secured a Westminster Hall debate on the UK's compliance with the Genocide Convention in June 2025,^[58] has repeatedly questioned ministers on the war in Gaza,^[59] and has called for a full arms embargo on Israel.^[60][61] In March 2026, Chowns urged the UK government to prevent energy companies from profiting from rising energy prices during the Iran war.^[62] She has also criticised the UK government's investment in nuclear weapons, arguing that billions of pounds would be better spent on other priorities.^[63]

On social care, she has pressed for cross-party action on reform,^[64][65] saying that "we need social care to work much more like the NHS, so that whether you get cancer or dementia, you get the care you need regardless of ability to pay".^[66] She has also campaigned for proportional representation,^[67] votes for 16-year-olds,^[68] and has advocated modernising the Commons^[69] by introducing electronic voting,^[70] reforming procedures and reducing the adversarial nature of Westminster; including by introducing a European Parliament-style hemicycle.^[71][72]

In 2025, Chowns criticised the UK government's proposed asylum reforms, describing them as "inhumane".^[73] She has also raised concerns in Parliament about inadequate public transport links in Herefordshire, after students were forced to leave college courses due to unreliable and costly services.^[74]

On 11 May 2025, incumbent co-leader Adrian Ramsay launched a joint leadership bid alongside Chowns for the 2025 party leadership election.^[75][76] Their supporters included Caroline Lucas, Jenny Jones, Molly Scott Cato, Jonathon Porritt and Hugh Fearnley-Whittingstall. The results of the leadership were announced on 2 September 2025, with Chowns and Ramsay being defeated by Zack Polanski.^[77]