Having joined the Labor Party as a student, Kerin was elected to the House of Representatives for the seat of Macarthur at the 1972 election. He remained on the backbench throughout the Whitlam Government, but lost his seat in Labor's landslide defeat at the 1975 election, returning to work at the ABARE. He was later selected as Labor's candidate for the safe seat of Werriwa at the 1978 by-election caused by the retirement of former Prime Minister Gough Whitlam, who had held the seat since 1952.