Dennis Joe Reimer was born on 12 July 1939 and grew up in Medford, Oklahoma. In 1962, he graduated from the United States Military Academy, was commissioned a second lieutenant, and graduated from the field artillery officer orientation course at Fort Sill. He married Mary Jo Powers that year as well.
Reimer completed Ranger and Airborne training from 1962 to 1963, and served with the 20th Artillery, 5th Infantry Division (Mechanized). In 1963 he was made a temporary first lieutenant. From 1964 to 1965, he served as an assistant battalion advisor, Advisory Team 60, United States Military Assistance Command, Vietnam. In 1965, he was promoted to permanent first lieutenant and temporary captain. From 1967 to 1968, he was an aide-de-camp to the commandant of the Armed Forces Staff College.
In 1968 and 1969, Reimer was promoted to temporary major and permanent captain, respectively. From 1968 to 1970, Reimer was an executive officer and S-3 of 2d Battalion, 4th Artillery, 9th Infantry Division, in Vietnam. Returning to the United States, Reimer taught at the U.S. Army Field Artillery School, and then attended the Command and General Staff College. From 1972 to 1974, he served as an aide to Army Chief of Staff General Creighton W. Abrams, Jr. In 1975 and 1976, he was made a temporary lieutenant colonel and permanent major, respectively.
From 1976 to 1978, Reimer commanded the 1st Battalion, 27th Artillery, 4th Infantry Division (Mechanized); he then was the commandant of the Training Command, 4th Infantry Division (Mechanized), at Fort Carson. He graduated from the Army War College in 1979 and was promoted to temporary colonel the same year. Reimer earned a Master of Arts from Shippensburg State College in 1979 as well. From 1979 to 1980, Reimer served with the V Corps Artillery, U.S. Army, Europe. He then commanded the Division Artillery, 8th Infantry (Mechanized), U.S. Army, Europe, until 1982. In 1982, he was promoted to permanent colonel.
Reimer returned to the United States to become the deputy assistant commandant of the Field Artillery Center and School at Fort Sill until 1984. That year, he was promoted to permanent brigadier general. He then served as the commanding general of III Corps Artillery. Reimer then successively served as chief of staff, U.S. Army Element, Combined Field Army, Republic of Korea, and assistant chief of staff, C3/J3 of the Republic of Korea/U.S. Combined Forces Command. In 1987, he was promoted to major general. From 1988 to 1990, Reimer commanded the 4th Infantry Division (Mechanized); in 1990, he received his third star.
From 1990 to 1991, he served as deputy chief of staff of the Army for operations and plans. In 1991, he was promoted to general and served as vice chief of staff until 1993. Reimer then commanded the United States Army Forces Command at Fort McPherson until 1995. He served as Army Chief of Staff from 20 June 1995 to 22 June 1999. Reimer retired from active duty in 1999.