Brigadier General Donnelly's military career began in 1984 when he was directly commissioned as First Lieutenant in the Medical Service Corps. He was assigned as an Immunologist to the Department of Pathology and Area Lab Services, as an Individual Mobilization Augmentee. In 1987, he was assigned as Field Medical Assistant and in 1988 as Commander of the Medical Detachment (Blood Bank: Service Headquarters – AJ) in Folsom, Pennsylvania. The 379th Medical Detachment was mobilized for Operation Desert Shield and deployed to Saudi Arabia in December 1990. Donnelly served as Commander of the 379th and as Army Component Blood Program Officer until the unit redeployed in April 1991. He continued to serve as Commander of the 379th after it redeployed until its inactivation in 1992. In 1992 he was named S-2/3 of the 338th Medical Group in Folsom, PA, a position he held until 1995. In 1995, BG Donnelly became the S-2 of the 424th Medical Battalion Logistics in Pedricktown, New Jersey. In 1997, he was named Executive Officer of the 338th Medical Group, a position he held until 1999.
After moving from the Philadelphia area to the San Francisco Bay area, BG Donnelly became G-3 of the 2nd Medical Brigade in San Pablo, California in 1999. In 2002, Donnelly became Chief of Staff of the 2nd Medical Brigade. The 2nd Medical Brigade was mobilized in support of Operation Iraqi Freedom 2 in December 2003. In January 2004, Donnelly deployed to the Iraqi Theater of Operations as Medical Brigade Chief of Staff. After redeploying from Iraq in 2005, BG Donnelly transitioned from the Chief of Staff position to the position of Commander, Medical Area Readiness Support Group (WE-MARSG), San Pablo, California in October 2005. After relinquishing command in 2007, he remained in the unit as a Project Officer, while working in Europe with his civilian employer. In June 2008, BG Donnelly served as Deputy Commander, 807th Medical Deployment Support Command, Seagonville, Texas and was promoted to Brigadier General in January 2009. In August 2011, he was assigned to the 3rd Medical Deployment Support Command in Forest Park, Georgia as Deputy Commander Operational Command Post.
BG Donnelly's military education includes Army Medical Department Officer Basic Course (1984), Army Medical Department Officer Advanced Course (1986), US Army Command and General Staff College (1995), and US Army War College (2002). He holds a Bachelor's degree in Biology and a Ph.D. degree in Immunology from the University of Pennsylvania and a Master's degree in Strategic Studies from the US Army War College.
BG Donnelly's military decorations include: the Legion of Merit, the Bronze Star Medal W/ 1 OLC, the Meritorious Service Medal (with Oak Leaf Cluster), the Army Commendation Medal W/2 OLC, and the Army Achievement Medal. He is a member of the American Association of Immunologists' the American Society of Microbiologists, and the American Association of Blood Banks.