LtCol Timothy Riemann was born in Conroe, Texas, and graduated with distinction from The Virginia Military Institute with a degree in history in 2005. He completed The Basic School in 2006 with Charlie Company 3-06 and then attended the Assault Amphibian Vehicle (AAV) Officer Course in 2007. 

Riemann was then assigned to 3rd Assault Amphibian Battalion in Camp Pendleton, California. During this time, he deployed as a provisional infantry platoon commander with Charlie Company to Al-Anbar Province, Iraq, from October 2007 to April 2008. Upon returning from Iraq, Riemann was selected to be the AAV platoon commander supporting Lima Company, 3rd Battalion, 5th Marine Regiment and deployed in January 2009 as a part of the 31st Marine Expeditionary Unit in Okinawa, Japan. 

In September 2009, Riemann returned to Quantico, Virginia, as a bullpen instructor at The Basic School. He served as a primary instructor for the Military Operations in Urban Terrain (MOUT) package, did two tours as a staff platoon commander and, in his final year at The Basic School, returned to the bullpen as an instructor. In 2012, Riemann was selected to attend resident Expeditionary Warfare School and, upon completion in 2013, returned back to 3rd Assault Amphibian Battalion.  

During his tour of duty at 3rd Assault Amphibian Battalion, Riemann spent 30 months as a company commander for both Headquarters and Service Company and Bravo Company. As the commanding officer of Bravo Company, he deployed in support of the Unit Deployment Program to Okinawa, Japan, and upon return went back to 3rd Assault Amphibian Battalion, completing his remaining eight months as the Battalion S-4 officer.

In 2016, Riemann was selected to attend resident Command and Staff College in Quantico, Vriginia, and upon completion of the course was assigned as the staff secretary for the Combat Development and Integration Commanding General. In May 2018, he helped establish the Brute Krulak Center for Innovation and Creativity at Marine Corps University and served as the operations officer during its inaugural year. In 2019, Riemann was selected to attend the Marine Corps School of Advanced Warfighting, completing the course as a commendatory graduate in the fall of 2020, earning the 0505 MOS.

In the fall of 2020, Riemann reported to 5th Marine Expeditionary Brigade/Task Force 51 in Manama, Bahrain, where he served as a Marine Air Ground Task Force Planner. In 2022, he was selected to be the inspector-instructor for 4th Assault Amphibian Battalion in Tampa, Florida, and upon completion of his command tour reported for duty as the chief of plans, U.S. Marines Central Command. In July 2025, Riemann reported to Yale University as the Marine Corps Fellow.