After being commissioning as a U.S. Army officer, Reimer attended the field artillery officer orientation course at Fort Sill. He served as assistant executive officer and executive officer, 20th Artillery, 5th Infantry Division (Mechanized) from 1963 to 1964. He was promoted to temporary first lieutenant, December 1963 followed by an assignment as assistant battalion adviser, Advisory Team 60, U.S. Military Assistance Command, Vietnam from 1964 to 1965. He was promoted to permanent first lieutenant in June 1965 and temporary captain in November 1965. He then returned to CONUS to attend the artillery officer advanced course at Fort Sill and Fort Bliss from 1965 to 1966. Reimer next commanded Company C, 11th Battalion, 3d Brigade, U.S. Army Training Center, Fort Benning from 1966 to 1967.
His next assignment was aide-de-camp to the commandant, Armed Forces Staff College, Norfolk, Virginia, 1967–1968. He was promoted to temporary major in September 1968 and permanent captain in June 1969. He served as executive officer and S–3, 2d Battalion, 4th Artillery, 9th Infantry Division, U.S. Army, Vietnam, 1968–1970 and as an instructor at the U.S. Army Field Artillery School at Fort Sill in 1970. Reimer attended the U.S. Army Command and General Staff College from 1970 to 1971 followed by an assignment as personnel management officer, Assignment Section, Field Artillery Branch, Office of Personnel Operations, Washington, D.C.
His next duty was as assistant executive officer and aide to the chief of staff, U.S. Army, General Creighton W. Abrams, Jr. from 1972 to 1974. He served as executive officer and S–3 (Operations and Training), Division Artillery, 4th Infantry Division (Mechanized), Fort Carson, from 1975 to 1976. During this time he was promoted to temporary lieutenant colonel and permanent major, June 1975 and June 1976, respectively. He commanded 1st Battalion, 27th Artillery, 4th Infantry Division (Mechanized), Fort Carson from 1976 to 1978, followed by assignment as commandant, Training Command, 4th Infantry Division (Mechanized).
Reimer attended the U.S. Army War College from 1978 to 1979, and received his Master of Arts degree from Shippensburg State College in 1979. He was promoted to temporary colonel in August 1979 and served as deputy commander and later special assistant to the commander, V Corps Artillery, U.S. Army, Europe, 1979–1980. Reimer was chosen to command Division Artillery, 8th Infantry Division (Mechanized), U.S. Army, Europe, 1980–1982 and was promoted to permanent colonel in March 1982. He moved on to become the division’s chief of staff from 1982 to 1983. He returned to Fort Sill as deputy assistant commandant, Field Artillery Center and School from 1983 to 1984. He was promoted to permanent brigadier general in September 1984 and took up assignment as commanding general, III Corps Artillery, Fort Sill, from 1984 to 1986.
He served as chief of staff, U.S. Army Element, Combined Field Army, Republic of Korea 1986 followed by assistant chief of staff, C3/J3, Republic of Korea/U.S. Combined Forces Command, 1986–1988. He received promotion to major general in September 1987 and commanded the 4th Infantry Division (Mechanized) at Fort Carson from 1988 to 1990. This was followed by promotion to permanent lieutenant general in July 1990 and assignment as deputy chief of staff for operations and plans, U.S. Army, and Army senior member, Military Staff Committee, United Nations, Washington, D.C., from 1990 to 1991. He received his fourth star in June 1991, serving as vice chief of staff, U.S. Army from 1991 to 1993. He then served as commanding general, U.S. Army Forces Command, Fort McPherson, from 1993 to 1995 before being selected as Chief of Staff of the U.S. Army. He has been credited with revamping the Army during a period of high-tempo deployments, including peacekeeping in Bosnia-Herzegovina and Kosovo. Reimer retired from active service on August 1999.
The U.S. Army has named their digital library after him.