Justice Kennett graduated from the Australian National University with a Bachelor of Arts (Hons) in 1985, a Bachelor of Laws (Hons) in 1988 and a Master of Public Law in 1993. His Honour also studied at the University of Sydney, graduating with a Master of Law in 2013.

Justice Kennett commenced his career in the Australian Public Service. His Honour held various roles, including as counsel assisting the Solicitor-General of the Commonwealth, before he commenced practice at the NSW Bar in 1998. He was appointed Senior Counsel in 2010.

At the Bar Justice Kennett specialised in administrative, constitutional, customs, native title, taxation and competition and consumer law matters. His Honour served for several years as chair of the Administrative Law Committee of the Law Council of Australia and a member of its Constitutional Law Committee.

From March to December 2022 Justice Kennett was a Resident Judge of the Supreme Court of the ACT. His Honour is a Fellow of the Australian Academy of Law.