Thomson was elected as the National Secretary of the Health Services Union (HSU) in 2002. In 2007 Thomson was elected as the Australian Labor Party (ALP) candidate for the seat of Dobell on the New South Wales central coast. He was re-elected in 2010. This election resulted in a hung parliament, with the Labor government of Julia Gillard and the Liberal/National opposition led by Tony Abbott each winning 72 seats in the 150 member parliament. Gillard retained power with the support of four of the six cross-bench MPs.