Kerr was the Labor candidate in the Division of Braddon in the 1977 Australian federal election, losing to future Premier of Tasmania Ray Groom.

Prior to entering politics, Kerr acted as Crown Counsel in the Tasmanian Solicitor-General's Department, as lecturer in constitutional law and Dean of the Faculty of Law at the University of Papua New Guinea, and as Principal Solicitor for the Aboriginal Legal Service of New South Wales.

On 14 December 2009, Kerr resigned his appointment as Parliamentary Secretary for Pacific Island Affairs and indicated he intended to return to legal practice.

[2] Upon Kerr's retirement, the previously safe Labor seat of Denison was won by Andrew Wilkie, an independent.

In 2010, Kerr became a founding member of Michael Kirby Chambers in Hobart where he practised as a barrister specialising in public, constitutional, administrative, refugee and human rights law, and appellate work.

In 2015, with the consent of the Australian Government, he was appointed by the Independent State of Papua New Guinea as its nominee as an arbitrator in a proceeding in the International Centre for the Settlement of Investment Disputes (ICSID).

Kerr ceased to serve as a Judge of the Federal Court of Australia on 26 February 2022 upon reaching the statutory retirement age.