- Organized and Federally recognized 1 February 1968 in the Oklahoma Army National Guard as the 120th Engineer Battalion, a non-divisional unit, in Eastern Oklahoma, with Headquarters at Okmulgee (organic companies organized from existing units)
- Ordered into active Federal service 7 December 2003 at home stations; released from active Federal service 3 June 2005 and reverted to state control
- Reorganized 1 September 2008 to consist of the Headquarters Company and the Support Company (concurrently; Company A reorganized and redesignated as the 2120th Engineer Company; and Company B as Detachment 2, 120th Engineer Company - hereafter separate lineages)
- Location of Headquarters changed 1 July 2011 to Broken Arrow
- Ordered into active Federal service 3 July 2012 at home stations; released from active Federal service 6 August 201 3 and reverted to state control
HOME AREA: Eastern Oklahoma
Headquarters Company (Broken Arrow), and Support Company (Broken Arrow), each additionally entitled to: