In addition to his service as a legislator, McMullen is employed in the private sector with a defense consortium developing prototype systems for the U.S. Navy. Prior to that, he worked for the federal government in the General Services Administration and the Department of Defense as a Contract Specialist and a Property Manager. McMullen also served in the U.S. Army as a paratrooper, seeing combat with his unit in Afghanistan.

A graduate of Blue Springs High School, McMullen obtained his bachelor's degree and a Master of Public Administration from the University of Missouri - Kansas City. He also obtained a Master of Legal Studies focused in Law from Washington University in St. Louis.

Born in Tulsa, Oklahoma, McMullen grew up in Independence where he currently resides. He and his wife have two children.