Duncan was first elected as a Member of the Legislative Assembly (MLA) for Weyburn-Big Muddy in a 2006 by—election, defeating Liberal David Karwacki with 49% of the vote.

[2] In a farewell speech to the Assembly, Duncan stated that he had achieved his main objectives in office and wanted to spend more time with his family.

[5] As Minister of Health, a portfolio he held for four years, Duncan passed legislation introducing private MRI clinics and CT scans to the province.

Duncan held the environment portfolio at a time when the province was resisting federal planning to address climate change, and in particular the imposition of carbon pricing.

"[14] The case ultimately progressed to the Supreme Court, where in 2021 the Act was ruled constitutional, which resulted in Saskatchewan having to adopt a carbon pricing scheme.

[22] The Opposition education critic, Carla Beck, called for Duncan's resignation in January 2022 after it emerged that he lied about consulting with school divisions about delaying the start of the semester, as Saskatchewan was experiencing a new COVID wave with record case counts and high levels of hospitalization driven by the omicron variant.

[26][27] Just weeks before the start of a new school year, in August 2023, Duncan unveiled new policies placing restrictions on sexual health education in Saskatchewan schools—including a ban on third party sexual health education—and requiring parental consent for children under the age of 16 who wished to change their preferred names or pronouns while at school.

[29][30] The assistant deputy minister of education reported receiving 18 letters over the summer of 2023 that were in support of a similar policy that was introduced in New Brunswick.

[32][33] On September 28, a Regina Court of King's Bench Justice granted an injunction against the policy, stating that "the protection of these youth surpasses that interest expressed by the government, pending a full and complete hearing.