Professor Donnellan served for 30 years as a Marine Corps officer with command assignments at the platoon, company, infantry battalion and training installation level. He commanded 2nd Battalion, 3rd Marines on combat deployments to both Afghanistan and Iraq. He served as officer in charge, II Marine Expeditionary Force Special Operations Training Group and commanded U.S. Marine Corps Mountain Warfare Training Center. He served as the deputy commander of U.S. Marine Corps Forces, Europe and Africa. His staff assignments included deputy Strategy Division Chief, U.S. Central Command; AC/S G5, U.S. Marine Corps Forces, Europe and Africa; and AC/S, C3, Regional Command Southwest, Afghanistan.