Edward P. Grimes was born in Dover, New Hampshire. He relocated to Massachusetts as a young man and officially entered the military in Boston. By 1879, he was serving as a Sergeant with Company F of the 5th U.S. Cavalry during the height of the Indian Wars.

In late September 1879, Sergeant Grimes was deployed to the Milk River region of Colorado during the White River War. His unit was engaged in a grueling conflict against Ute warriors, a confrontation that quickly escalated into a dire siege. From September 29 to October 5, the command found itself in a desperate tactical position.

The 5th Cavalry was nearly depleted of ammunition and was surrounded on three sides by enemy forces. The situation required a high-risk resupply effort to prevent the line from collapsing. Despite the immediate danger, Sergeant Grimes took the initiative to rectify the shortage.

Grimes voluntarily moved to bring up a fresh supply of ammunition to the front lines. He performed this task under heavy, sustained fire at point-blank range. His actions ensured the command could maintain its defense throughout the week-long engagement. For his gallantry during this action, he was later awarded the Medal of Honor.