Chamberlain joined the Liberal Democrats after the 2015 United Kingdom general election, which saw the party lose 49 of its 57 MPs. After standing in an "unwinnable" seat in the 2017 Scottish local elections, Chamberlain was asked by Scottish Liberal Democrat leader Willie Rennie to consider putting herself forward to stand in the 2017 general election. Chamberlain stood as a paper candidate in Stirling finishing fourth in the constituency, winning 3.4% of the vote with a small increase in vote share and absolute votes as compared to the result in 2015. Chamberlain was selected as the prospective Parliamentary candidate for the Liberal Democrat target seat of North East Fife in June 2018, and in March 2019 was appointed as the Scottish Liberal Democrats' spokesperson for Constitutional Relations by Willie Rennie. Chamberlain stood for the party in the 2019 general election, comfortably overturning the Scottish National Party's slim two-vote majority in North East Fife by winning 1,316 more votes than the incumbent, Stephen Gethins. With the Liberal Democrat contingent in the UK Parliament significantly diminished following a poor general election result, Chamberlain was appointed as party spokesperson in three areas: Political & Constitutional Reform; Scotland, Wales and Northern Ireland; and International Development. In a wide-ranging maiden speech, Chamberlain spoke about her constituency, gender equality, the European Union, and electoral reform, while also paying tribute to her predecessors. During the course of the COVID-19 pandemic, Chamberlain expressed her belief that "civil liberties mustn’t be curtailed more than necessary, and the powers mustn’t be used disproportionately against minority communities", and called for vigilance "to ensure that those powers are used properly and evenhandedly". Chamberlain wrote a letter calling for the resignation of Catherine Calderwood, the then Chief Medical Officer for Scotland, following reports that Calderwood had failed to follow coronavirus guidelines by visiting her second home, which was located in Chamberlain's constituency. In April 2020, following the decision to limit the number of MPs sitting in the House of Commons, Chamberlain led a group of opposition MPs in calling for the establishment of a COVID-19 select committee. Chamberlain stated that the creation of such a committee was "the only way to guarantee smaller parties from across the political spectrum the opportunity to scrutinise and ask questions of Ministers at this critical time". Chamberlain was one of three Liberal Democrat MPs to endorse Layla Moran in her campaign to become leader of the party.
Personal life
Though originally from Greenock, Chamberlain has lived in Fife since 2003. She is married, and has two children with her husband, Keith, who is a member of the Scottish National Party. Chamberlain has stated that her father encouraged her to get involved in politics, and recalled his words in an interview with The Herald in 2020: "When I found out you were getting involved in all this politics stuff I thought you were aff yer heid, but then I look at those eejits on the telly and I think you can be just as good as them".