Lieutenant Colonel Johnsen grew up in Frankfort, Illinois. He was commissioned as a Second Lieutenant after graduating from the United States Naval Academy in 2006 with a Bachelor of Science in Aerospace Engineering.
Upon commissioning, he attended The Basic School in Quantico, Virginia, followed by flight training at NAS Whiting Field, Florida. He was designated a Naval Aviator in October 2008 and reported to HMM(T)-164 at MCAS Camp Pendleton, California, for training in the CH-46E.
After qualifying as a Helicopter Second Pilot in the CH-46E Sea Knight, LtCol Johnsen’s first operational assignment was with Marine Medium Helicopter Squadron 165 (HMM-165) from May 2009 to April 2011. During this time, he served as a schedule writer and the squadron social officer, and participated in humanitarian relief operations in Pakistan during the summer of 2010 as part of the 15th MEU. Upon returning from deployment, he transitioned with the squadron to the MV-22B, and the unit became Marine Medium Tiltrotor Squadron 165 (VMM-165) in April 2011.
From September 2011 to July 2016, LtCol Johnsen held various positions in VMM-165, including social officer, communications officer, future operations officer, maintenance officer, quality assurance officer, and assistant operations officer. He deployed with the squadron to Afghanistan in support of OPERATION ENDURING FREEDOM and to Kuwait in support of Special Purpose Marine Air Ground Task Force-Crisis Response-Central Command for OPERATION INHERENT RESOLVE.
In August 2016, LtCol Johnsen was assigned to Marine Helicopter Squadron One as a tiltrotor pilot, where he served as a schedule writer, the squadron social officer, and a flight officer. After completing his tour at Marine Helicopter Squadron One, he was assigned to Marine Medium Tiltrotor Squadron 264.
At Marine Medium Tiltrotor Squadron 264, LtCol Johnsen served as the operations officer from September 2019 until the squadron's deactivation in June 2020. He then assumed the role of operations officer and later aviation maintenance officer at VMM-261. In March 2021, he deployed in support of North and West Africa Response Force Air Combat Element 21.2 as the executive officer and maintenance officer. Upon returning, he took on the roles of Personnel Support Detachment Officer-In-Charge and campaign plan officer at Marine Aircraft Group 26.
From July 2022 to June 2025, LtCol Johnsen has served as the European Future Operations Officer for Marine Forces Europe/Africa.
LtCol Johnsen is a graduate of the Marine Corps Distance Expeditionary Warfare School, Marine Corps Distance Command and Staff College, the Weapons and Tactics Instructor course, and holds a Master of Public Affairs from Indiana University's O’Neill School.
His personal decorations include the Meritorious Service Medal, the Air Medal with four strike/flight awards, the Navy and Marine Corps Commendation Medal, the Navy and Marine Corps Achievement Medal, and the Presidential Service Badge.