Shaar graduated from the U.S. Naval Academy and was commissioned a second lieutenant in the Marine Corps where she served for more than 30 years. She earned a Master of Science in management from the Naval Postgraduate School; a Master of Arts in strategic studies from the Army War College; and a Master of Science in finance at Georgetown University.

She served as a financial management officer in numerous operational and support commands throughout her career. Her leadership assignments included headquarters and service battalion commander at Marine Corps Recruit Depot San Diego; commander of Region-1 (Eastern Europe and Eurasia) within the Marine Corps Embassy Security Group; and service company commander supporting U.S. Marine Corps Forces, Atlantic.

She deployed as a civil military affairs officer in support of Operation Iraqi Freedom with 1st Marine Division and served as a congressional appropriations liaison representing the Marine Corps on Capitol Hill. Additionally, she completed multiple headquarters tours within Headquarters, Marine Corps, the Department of the Navy, and the Pentagon.