From 1994 to 1998, Duncan worked as a research officer for Ross Ainsworth, a long-serving Nationals MLA for the seat of Roe. She joined the party herself in 2001, and from 2002 to 2003 served as president of the party's Esperance branch. Duncan served on the party's state executive from 2003 to 2004, and then from 2004 to 2008 served as state president, becoming the first woman to hold the position. During her presidency, she was one of the architects of the party's Royalties for Regions policy. She first stood for parliament at the 2005 state election, running second on the party's ticket in Agricultural Region, but was not elected. She was also second on the party's ticket for the Senate at the 2007 federal election, but neither her nor the lead candidate, Tony Crook, were elected. However, in January 2008, Murray Criddle resigned from parliament, with Duncan taking his position in the Legislative Council on a countback. In her inaugural speech, she expressed concerns about foreign ownership of Australian resources, economic rationalism, the dominance of the two major parties and the neglect of regional and rural areas. At the 2008 state election, Duncan transferred to the Mining and Pastoral Region, allowing a former party leader, Max Trenorden, to take her old seat. She was subsequently included in the new ministry formed by Colin Barnett of the Liberal Party, becoming parliamentary secretary to the Minister for Regional Development and Minister for Lands, and also assistant minister to the Minister for State Development and the Minister for Transport. In April 2012, Duncan won preselection for the seat of Kalgoorlie at the 2013 state election. At the election, in April 2013, she received 56.3 percent of the two-candidate-preferred vote, becoming the first member of the party to win the seat since its creation in 1901. After the election, she was elected deputy speaker to Michael Sutherland. Duncan stood for the deputy leadership of the National Party in November 2013, following the resignation of party leader Brendon Grylls, but was defeated by Mia Davies. Davies was also Grylls' replacement in cabinet. In December 2014, Duncan stated that she might consider becoming an independent, having being overlooked a second time for a ministerial position during a cabinet reshuffle. She also warned her party not to "get too focused on agriculture. Royalties for Regions comes from mining and petroleum royalties, so don't turn your back on the regions they come from." However, she re-affirmed her commitment to the party shortly after. Duncan announced in December 2015 that she would retire from parliament at the 2017 state election.