By that time Harold was working at Garst Brothers Dairy in Roanoke, Virginia. Harold married Mary Katherine Goggin on 17 Sep 1930 in Roanoke where they lived. Although he was 21 he was still living in his now widowed mother's household. In order to supplement his income Harold had enlisted in the Virginia Army National Guard on 18 Aug 1925. He rose through the ranks being promoted from PVT to PFC to CPL to SGT to 1SG and then MSG before being commissioned a 2LT on 15 Mar 1938 and being federally recognized at that rank on 28 Jan 1939. He was serving in Service Company 116thh Infantry at that time. The couple had a son in 1940. Harold was now manager of the dairy and doing very well owned the family home at 302 Guilford Avenue which was valued at $4000 and reported a 1939 income of $1540.
2LT Cassell was mobilized with his National Guard unit on 3 Feb 1941 and went with that unit to Fort George Meade, Maryland. He received a reserve commission as LTC on 5 Dec 1943 although he was not promoted to CPT with federal recognition until 18 Apr 1943. He was federally recognized as a LTC on 29 Jun 1950. After having served as regiment executive officer LTC Cassell took command of the 116th in November 1944 before command of the unit was given to LTC Sidney Bingham in early December 1945.
On 6 Jun 1944, D-Day, LTC Cassell was serving as the 116th's Executive Officer. He replaced the wounded LTC Tom Dallas as commander of 1st Battalion 116th Infantry on 5 Aug 1944 near Vire, France. On 10 Nov 1944 he assumed command of the regiment from COL Dwyer who was being transferred. He relinquished command to LTC Sidney Bingham at the beginning of December 1945 but would again command the regiment during its duties in the occupation of Germany after May 1945 and was still commanding the unit when it demobilized at Camp Kilmer, New Jersey in 1946. LTC Cassell was one of he only 50 men who had mobilized with the unit in February 1941 who were still with the regiment 5-years later. LTC Cassell was then assigned to the Fort Leavenworth where he and Mary would have a 2nd child, a daughter, in April 1949. After being reassigned in 1949 the family lived in Anne Arundel, Maryland. As a LTC, Harold was paid about $5600 in 1949. Then he had an assignment in Japan and in the Republic of Korea before returning to Fort Benning and Columbus, Georgia where he commanded the 1st Battalion of the School Brigade at the Infantry School there until 1957. LTC Cassell then served a tour in Tuscaloosa, Alabama where he was Chief of the Tuscaloosa Military Sub-district. He was then assigned as commander of the Western Area Command, an Army Reserve unit. LTC Cassell retired from the Army on 30 Jun 1961 and lived in Columbus, Georgia.
LTC Cassell died 22 Jan 1974 in Columbus, Georgia. He rests forever in the Fort Moore Main Post Cemetery at Fort Benning, Georgia. His awards include the Silver Star, 3 Bronze Stars (BSM with 3 OLC) and the Army Commendation Medal with Oak Leaf Cluster.