Colonel William (Trey) S. Chairsell III was commissioned a 2ndLt in the USMC following completion of the NROTC program at Norwich University in 1999. 2ndLt Chairsell completed Logistics Officer Course in 2000 and was subsequently assigned as the Maintenance Management Officer for Battalion Landing Team (BLT) 3/1. He deployed with 13^th MEU (SOC) from August 2000- February 2001. Lieutenant Chairsell also served as the Motor Transport Officer and Ordnance Officer for 3/1. Upon transfer to BLT 1/1, 1stLt Chairsell assumed duties as the S4A and completed another deployment with 13^th MEU in support of Operation Iraqi Freedom (OIF). Capt Chairsell subsequently redeployed as the Regimental S4A for Regimental Combat Team-1 in support of OPERATION PHANTOM FURY in Fallujah, Iraq until Dec 2004.

Captain Chairsell completed his active duty in April 2005 and joined the SMCR as a Site Commander with 6^th Motor Transport Battalion in Jan 2006. In 2009, Capt Chairsell was mobilized as the Transportation Support Company Commander for Combat Logistics Battalion-46 and participated in OIF 9.2 which focused on USMC retrograde operations in Iraq. He was promoted to Major in December 2009. Following demobilization, Major Chairsell took a position as an Individual Mobilization Augmentee (IMA) with MARFORPAC G4 where he focused on exercises on the Korean peninsula.

In 2011, Major Chairsell attended the Marine Corps Command and Staff College where he was named a Distinguished Graduate. He subsequently joined the 4^th Marine Logistics Group Headquarters on ADOS in August 2012. During his time in the 4^th MLG G3, Major Chairsell managed various projects and initiatives including the development of the Annual Training plan, the Innovative Readiness Training program, the establishment of the Service Component Operation Logistics Augmentation Cell, and the initial Logistics Combat Element design for the Reserve Integrated Training Exercise. During this assignment, Maj Chairsell was selected for the Active Reserve program.  In March 2015, Major Chairsell assumed duties as the Deputy Assistant Chief of Staff G4 and was promoted to LtCol in December 2015.

LtCol Chairsell attended the National War College from August 2016-Jun 2017 where he received a Master of Science in National Security Strategy. Following Top Level School, he was selected to be the Inspector-Instructor, 4th Medical Battalion, 4^th Marine Logistics Group. In this assignment, he championed the Commanding General’s innovation focus of effort bringing new and developing technologies to the critical health service support mission. Upon completion of command, he assumed the role of Reserve Liaison Officer for I MEF where he directed the largest operationally focused  Individual Mobilization Augmentee program in the Marine Corps. He was promoted to Colonel on 1 July 2021 and assumed the role of Director, Marine Corps Individual Reserve Support Activity. During this tour, he revamped how Individual Ready Reserve Marines are kept informed of opportunities and obligations with a focus on the talent management line of effort.

In July 2023, Colonel Chairsell assumed the role as Chief of Staff for 4th Marine Division.