Dennis was president of the Humanist Society of WA from 1998 and continued in that role until he stood down after a stroke in 2006. He was an enthusiastic participant and worker in all the activities and meetings of the Society.
Dennis and his wife Ann arrived from South Africa in 1988. They had found 'apartheid' objectionable and were members of a Freethinkers group there. He joined the WA Humanist Society in 1988 and Ann joined soon after.