In July 2018, Thwaites won Labor preselection for the Division of Jagajaga, following Macklin's retirement. According to The Guardian, her victory was "almost a direct result of Josh Burns' win in Macnamara, with the Labor left concerned about its female representation".[7] She retained Jagajaga for the ALP at the 2019 federal election with a small positive swing.[5] Thwaites was returned as the member for Jagajaga at the 2022 Federal Election with an increased margin of over 62% of the two party preferred vote.[8]
In 2021, Thwaites co-authored a book, Enough Is Enough, with her predecessor Jagajaga member, Jenny Macklin.[9]
In April 2023, Thwaites co-signed an open letter to Prime Minister Anthony Albanese calling for an urgent boost to JobSeeker, Youth Allowance, and other support payments.[10] In the same month she joined others advocating for an expansion of the single parenting payment to include more mothers.[11]
In July 2024 she was appointed Assistant Minister for Ageing, for Social Security and for Women.