During the Second World War he was attached to RAF Coastal Command, where he was involved in experiments on escape from submarines, one of which left him with permanent lung damage.
After the war Rendel moved to Edinburgh to do animal genetics research at Edinburgh University, where he was jointly in charge of a project on dairy research, establishing the fundamental principles of artificial insemination in dairy progeny-testing programmes.
In 1951 he travelled to Australia to join CSIRO and establish a team at the University of Sydney to teach animal genetics and develop a research programme into animal breeding methods for domestic fowl, sheep, dairy and beef cattle.
His personal interest was in breeding cattle to build up the livestock industry in the tropical and sub-tropical regions.
After retirement in 1980, Rendel moved to Drinkstone Green in Suffolk, England, where he bred Booroola sheep imported from Australia.