From 1981 to 2001 Chief Justice Allsop practised at the Bar in New South Wales and elsewhere in Australia. He was appointed Senior Counsel in New South Wales in 1994 and Queen’s Counsel in Western Australia in 1998.
From 7 May 2001 to 1 June 2008 he served as a Judge of the Federal Court of Australia, undertaking the roles of trial and appellate judge on a full range of Federal Court work. From 2 June 2008 to 28 February 2013, Chief Justice Allsop was President of the New South Wales Court of Appeal. He was appointed Chief Justice of the Federal Court of Australia as of 1 March 2013.
From 1981 to 2014 Chief Justice Allsop taught part-time at the University of Sydney as a tutor and lecturer in property, equity, bankruptcy, insolvency, corporate finance and maritime law. From 2015 to 2018 he taught maritime law part-time at the University of Queensland. From 2005-2009, he was a member of the board of World Maritime University in Malmö, Sweden. From 2008 to 2011 he was a member of the Board of the Australian Maritime College. On January 2010, he was elected as an Honorary Bencher of the Middle Temple. On 19 March 2013 he was elected a member of the American Law Institute. He is a Fellow of the Australian Academy of Law. He is President of the Francis Forbes Society for Australian Legal History. From 2018 he was appointed Inaugural Patron of the Australian Insurance Law Association. From 2019 Chair of ACICA Judicial Liaison Committee. From 2019 he accepted membership on Asian Business Law Institute (ABLI) Board of Governors representing the Australian Judiciary.