Dennis was born and raised in eastern Pennsylvania. He received a BS in Zoology from Penn State and an MS in Water Chemistry from the University of Wisconsin. He worked for the US Geological Survey for more than 42 years studying the water quality of lakes and streams across the United States. Since retiring, he has focused on the aesthetic aspects of photography, rather than on the documentary aspects he had previously used to describe field study sites. He lives in Fort Collins with his wife, Betty, and their two dogs, Clancy and Fergus. He posts occasionally on Instagram @d_macdogall. The dogs post occasionally @macdogall_bros.