Steele-John holds the portfolios of Health, Disability Rights and Services, Foreign Affairs and Peace & Nuclear Disarmament for the Australian Greens.

On 27 October 2017, the High Court of Australia, sitting as the Court of Disputed Returns, ordered the Australian Electoral Commission to conduct a recount of senate ballots in Western Australia, and Steele-John was declared elected at age twenty-three, making him the youngest sitting member in the Australian parliament and youngest senator.

[6] Steele-John has previously held the Greens portfolios of Youth, Defence and Veterans’ Affairs, Digital Rights and I.T., Sustainable Cities, and Trade.

[8] In 2019, Steele-John's campaigning successfully led to the establishment of the Royal Commission into Violence, Abuse, Neglect and Exploitation of People with Disability.

In 2020, Steele-John successfully campaigned for a compensation scheme for thalidomide survivors, and in 2021 pressured the government to abolish independent assessments in the NDIS.

[citation needed] In 2020, Steele-John introduced the Defence Amendment (Parliamentary Approval of Overseas Service) Bill 2020, which seeks to ensure the decision to go to war is made by the Parliament, not the Prime Minister.

[13] An active anti-nuclear campaigner, Steele-John is a strong advocate for Australia's signing of the Treaty on the Prohibition of Nuclear Weapons, and to rethink the AUKUS alliance.

In February 2018, Steele-John expressed disappointment at the lack of government support for the Australian video game development industry.