Dennis was raised in Arizona. He graduated from the University of Wisconsin—Madison with a Bachelor’s degree in History and Political Science in 1981. He returned to Arizona where he received his Master’s (1991) and Ph.D. (1998) in History from Arizona State University. He has worked as the Corporate Historian and Archivist for Arizona Public Service Company, the photograph archivist at Arizona State University, the Corporate Archivist for Phelps Dodge Corporation, the Corporate Historian and Archives Manager for Sears, Roebuck and Co. in Chicago, and the Archives Manager for Toyota Motor Sales U.S.A. in Los Angeles before returning to Arizona in 2008. He began working for the State Archives in June 2012 and was promoted to Assistant Director History, Archives & Records Management in August 2013. He also serves as President of the Arizona Archives Alliance.