Before his appointment to the tribunal, Mensah was Ghana's High Commissioner to South Africa and a former Assistant Secretary-General at the International Maritime Organization (IMO).

Thomas Aboagye Mensah was born on 12 May 1932 in Kumasi, Gold Coast.

In addition, he worked from 1965 to 1966 as a judicial officer at the International Atomic Energy Agency in Vienna.

From 1 October 1996, he was a member of the newly founded International Maritime Tribunal in Hamburg.

He was a president of the IMO and defended the country in a number of lawsuits at the International Court on the Law of the Sea (ITLOS), the most notable one being the boundary proceedings between Ghana and Cote d'Ivoire.