Mullenix has been affiliated with John E. Reid & Associates since 1978. He has conducted thousands of interviews and interrogations and provided instruction in The Reid Technique® of Interviewing and Interrogation, including to U.S. Counterintelligence agents. Military Intelligence agents, Special Forces, and HUMINT collectors as well as federal, state, and local law enforcement officers. His publications include: "HUMINT Collection: A Look Through Non-coercive Field Questioning of Jihadist Combatants," Marine Corps Gazette Magazine, June 2014; "Interrogation Theme Selection for Jihadist Combatants," Special Warfare Magazine, April-June 2014; and "Efficacy of HUMINT Collection Protocols Under Army Field Manual 2-22.3 for Intelligence Interviewing and Interrogation of Terrorist Information Sources," white paper research submission, United States High-Value Detainee Interrogation Group, May, 2012. Mullenix has practiced law since 1983 and is licensed by the State of Illinois, the United States District Court for the Northern District of Illinois, and the United States Seventh Circuit Court of Appeals.