He became Premier following Askin's retirement from politics and held the position until he was replaced by Willis in a party vote.

Lewis was first elected to the New South Wales Legislative Assembly for the Electoral district of Wollondilly for the Liberal Party in 1957, and served until his resignation in 1978.

Askin announced his resignation late in 1974, and Lewis was chosen as leader over Willis and Justice Minister John Maddison.

At a caucus meeting on 20 January 1976, parliamentary backbencher Neil Pickard called a spill motion, which was carried 22 votes to 11.

[4] In opposition Lewis did not hold any position within the Shadow Cabinet and later resigned from Parliament on 7 September 1978, the vacancy becoming one of the justifications for calling the election for 7 October 1978.

[5] On his departure from parliament, he was permitted by Queen Elizabeth II, on the Governor's recommendation, to continue to use the title "The Honourable".