[3] Prior to 1996, he was a lecturer in law at the University of Cambridge and a fellow of Pembroke College.
In 1996, he left Cambridge and was appointed Bracton Professor of Law at the University of Exeter, and in 2010 Tettenborn left Exeter to join Swansea's Hillary Rodham Clinton School of Law.
[10][11] In 2022, following the capture by Russian forces of British-born Ukrainian soldiers Aiden Aslin and Shaun Pinner during the Russian invasion of Ukraine, Tettenborn commented that the statement Aslin and Pinner were fighting illegally in the country – by Northern Ireland Secretary Brandon Lewis and Shadow Attorney General Emily Thornberry – could be used by Russian forces to justify an atrocity against the soldiers.
Tettenborn said that it was incorrect as a matter of international law to say the two were fighting in the country illegally.
Later, the Prime Minister's spokesperson said that the British government does not consider the pair to have being fighting illegally in the war.