Erwin was elected for the Liberal Party as the member of the House of Representatives seat of Ballaarat at the 1955 election. He was Government Whip from February 1967 to February 1969 and strongly supported John Gorton's election as Liberal leader following the disappearance and presumed drowning of Prime Minister Harold Holt on 17 December 1967. On 13 February 1969 he was appointed Minister for Air, in the reshuffle following Paul Hasluck's appointment as Governor-General. Gorton subsequently appointed one of his young staffers, Ainsley Gotto as his private secretary and came to rely on her for political advice. In November 1969, Erwin was left out of Gorton's second ministry. Erwin's explanation for his dismissal was: Erwin chaired the Joint Select Committee on Parliamentary and Government Publications, which produced the first edition of the Australian Government Style Manual. He retired from parliament at the 1975 election. His final political candidacy occurred at the 1979 elections for the Australian Capital Territory House of Assembly, where he ran unsuccessfully as an independent.