In 1996, Thomson was elected to the seat of Wills, formerly a blue-ribbon Labor seat that had been held by Bob Hawke. However, it had become less safe for Labor. In 1992 the independent Phil Cleary won a by-election; and in 1993, although Labor retained government, Cleary again defeated the Labor candidate who received only 41.9% of the primary vote. In the 1996 election, in which Labor's support slumped badly and Paul Keating's government was destroyed, Thomson became the Labor candidate and proved himself a strong campaigner. He won the seat, gaining 50.0% of the primary vote. He has held the seat since then. In the 2007 election, which Labor won, Thomson achieved a swing, on the two-party-preferred vote of 5.5 points, and received 56.9% of the primary vote. In the 2010 election, which resulted in near defeat for Labor, Thomson like other Labor incumbents lost ground on the primary vote (51.81%); yet he achieved a further slight positive swing on the two-party preferred vote (0.24%).