Major General Shea enlisted in the Marine Corps in January 1985. Trained as a ground support equipment mechanic, she served with 2d Marine Aircraft Wing at Marine Corps Air Station Cherry Point, NC. She was commissioned from the U.S. Naval Academy in 1991 and was assigned as a communications officer.

Major General Shea's operational assignments have included: platoon commander and company commander with 2d Landing Support Battalion, 2d Force Service Support Group in Camp Lejeune, NC; company commander with Marine Wing Communications Squadron-38, 3d Marine Aircraft Wing, San Diego, CA; S-6 Officer for the 22d Marine Expeditionary Unit (Special Operations Capable) for deployments to Afghanistan and Iraq; Director of the Commander's Initiatives Group for the Commander, International Security Assistance Forces, Afghanistan; Assistant Chief of Staff G-6, 3d Marine Aircraft Wing, San Diego, CA; and Commanding Officer, I MEF Headquarters Group, Camp Pendleton, CA.

Her duties in the supporting establishment have included duty as a series commander, company commander, and battalion commander with 4th Recruit Training Battalion, and the Assistant Director of the Drill Instructor School, MCRD Parris Island, SC; as a White House Fellow and Special Assistant to the Homeland Security Advisor, The White House, Washington, DC; as a Strategic Analyst with the Strategic Initiatives Group, Headquarters, Marine Corps; as the Director, Commandants Staff Group, Headquarters, Marine Corps; as the Deputy Commandant of Midshipmen, U.S. Naval Academy, Annapolis, MD; and as Acting Deputy Assistant Secretary of Defense for East
Asia.

Major General Shea assumed her current duties as the Legislative Assistant to the Commandant of the Marine Corps in May 2021. Major General Shea's military education includes graduating as an honor graduate from both the Marine Corps Basic Communications Officers Course and the Amphibious Warfare School, and as the Commandant's Distinguished Graduate from the Industrial College of the Armed Forces, National Defense University. She holds a Masters degree in Computer Information Systems from Boston University (2000) and a Masters degree in National Resource Strategy from ICAF (2011).

Her personal decorations and awards include the Defense Superior Service Medal, Legion of Merit, Bronze Star, Meritorious Service Medal, the Joint Commendation Medal; Navy-Marine Corps Commendation Medal, Navy-Marine Corps Achievement Medal, the Combat Action Ribbon, and the Outstanding Volunteer Service Medal.