Napthine was elected to the Victorian Legislative Assembly in 1988 as Member for the country seat of Portland, as the electorate was then known. He was re-elected in 1992, 1996 and 1999. The electorate of Portland was abolished in the 2002 redivision. Napthine was then elected MLA for South-West Coast in the November 2002 election. He was re-elected in November 2006 with an increased margin.