Murnain worked as an electorate officer for Michael Daley MP from 2005–2007. She served as a policy advisor for Ageing and Disability Services to Kristina Keneally MP from 2007–2008. In 2008 Murnain began working at the NSW Labor Party head office as an organiser running campaigns in local, state and federal by-elections and general elections. With the support of the Australian Political Exchange Council, Murnain undertook an Individual Study Tour of the United States of America in 2012 and aided the Democratic Party campaign. In 2013 Murnain was appointed Assistant General Secretary of NSW Labor. In February 2016 Murnain was elected by ballot as the first female General Secretary of NSW Labor. Federal Opposition Leader, Bill Shorten, backed Ms Murnain, then aged 29, stating that "She's not to be underestimated. She'll make a strong NSW General Secretary." In response to a report into the status of women within the NSW division of the Labor Party, commissioned in response to the sexual harassment claim, Murnain stated that her goal as General Secretary is to achieve cultural reform and strengthen Labor's affirmative action rules. Murnain introduced the Party's first code of conduct and announced the introduction of compulsory ethics seminars for newly elected representatives. At the 2016 NSW Labor Conference delegates voted in affirmative action reforms such as a women's mentoring program and targets for women to have equal representation in all elected party positions, across all council, state and federal electorates, by 2027. In the lead up to 2016 federal election, Murnain led a campaign strategy similar to Obama's "organising to win" campaign. Labor NSW won 24 of the available 47 seats, with half of NSW Labor seats held by women, meeting the party's affirmative action goal. Murnain brokered a deal between left and right factions of NSW Labor to have Linda Burney, NSW Deputy Leader, run for the federal seat of Barton. Burney was subsequently elected as the Member for Barton and became the first indigenous woman elected to the Australian House of Representatives. Murnain oversaw the local governmentelections in 2016 where Labor regained most councils across western Sydney, and three by-elections in Canterbury, Wollongong and Orange. Murnain-backed Sophie Cotsis to run in Canterbury and Paul Scully in the seat of Wollongong. Murnain was suspended as General Secretary of NSW Labor on 28 August 2019 after she testified before the NSW Independent Commission Against Corruption to knowing, since September 2016, about a potentially illegal 2015 donation of $100,000 from Huang Xiangmo, a billionaire closely linked to the Communist Party of China, to the NSW Labor Party; and not disclosing the donation to relevant authorities. Murnain resigned as General Secretary of NSW Labor on 17 October 2019.