Harrell earned his commission as an Infantry officer through East Tennessee State University's Army ROTC program in 1973 and was assigned to the 2nd Battalion, 508th Infantry Regiment, 82nd Airborne Division, as a rifle platoon leader and as an anti-tank platoon leader.

In 1980 Harrell served as a company commander in the 1st Battalion, 505th Infantry Regiment, 82nd Airborne Division.

Harrell participated in the invasion of Grenada and afterwards served with the 10th Special Forces Group.

In 1985 Harrell volunteered for and completed a specialized selection and operator training course for assignment to the Army's 1st Special Forces Operational Detachment – Delta, publicly known as Delta Force, at Fort Bragg.

After graduating from the United States Army War College, Carlisle Barracks, Pennsylvania, in June 1995, Harrell was assigned as the Deputy Commander of Delta Force and commanded the unit from July 1998 to July 2000.