[1] Shannon joined the New Zealand Air Force in June 1942 as a radio mechanic, and served in Guadalcanal in the Solomon Islands.
[3][2] As a young man, Shannon was an avid butterfly collector, and before his posting in the Solomon Islands in World War II he consulted with entomologists at the Plant Diseases Division of the DSIR, who supplied him with cyanide for killing insects.
He converted large biscuit tins for butterfly storage by installing racks of card on which to pin his collections.
[3] After the war, he continued collecting well into his 80s, making expeditions to many tropical countries including Peru, Indonesia, Malaysia, New Guinea, Japan, and Costa Rica.
[3] Shannon was also an assiduous collector of reference books about Lepidoptera, and continued to purchase butterfly specimens from dealers after his declining health prevented further expeditions.