Fenner joined the Field Naturalists' Society (FNS), initially the Field Naturalists' Section of the Royal Society of South Australia, in June 1917. He was elected Vice-President in August 1917 and was President from 1919 to 1921. He was active on the Committee and at evening meetings of the Society, and conducted many of their field trips. He was a member of the group which in 1921 designed the badge of the 'Field Nats', with bright red Sturt's peas (Clianthus dampieri) as the emblem. He also served as the representative of the FNS on the Flora and Fauna Protection Society for many years.