Sergeant Major Sally O’Brien enlisted in the U. S. Marine Corps in October 1975.  She was assigned to various duty positions at Marine Barracks, Charleston, South Carolina and Marine Corps Recruit Depot, Parris Island, South Carolina where she was meritoriously promoted to Corporal and Sergeant.  Sergeant Major O’Brien was later assigned to the Marine Corps Development and Education Command, Quantico, Virginia where she was meritoriously promoted to Staff Sergeant after serving only 6 years and 3 months of active duty.  She was honorably discharged from the Marines in July 1982.

In August 1982, Sergeant Major O’Brien enlisted in the Massachusetts Army National Guard where she served as an Equal Opportunity Representative for the First Brigade, 26^th Infantry Division.  She was then accepted into the Active Guard Reserve (AGR) program and performed as a Production Recruiter and Recruiting Operations Sergeant and was promoted to Sergeant First Class in 1983.  After her tour in Recruiting Command she established the State Incentive and Bonus Program, followed by an assignment as a Training Non-Commissioned Officer and Senior Analyst in the Standard Installation Division Personnel System.  Sergeant Major O’Brien was then assigned to Officer Personnel Management where she received the distinction of being the first female promoted to the rank of Master Sergeant in the Massachusetts Army National Guard.  In November 1992, she was assigned to the Troop Command in Lexington, Massachusetts.

After the deactivation of the 26^th (Yankee) Infantry Division, she transferred to the 100^th Troop Command, California Army National Guard.  She subsequently served as the Senior Personnel Sergeant, Enlisted Personnel, and in 1997 was promoted and assigned as the first female Directorate Sergeant Major for Military Personnel, Office of the Adjutant General. Sergeant Major O’Brien was appointed to Command Sergeant Major on 1 October 1999 and was selected to serve as the inaugural Command Sergeant Major of the 49^th Combat Support Command.  She concurrently served as Directorate Sergeant Major, Military Personnel and Command Sergeant Major until her full-time reassignment in April 2001.  She served in this assignment until 1 April 2003 when she was selected to serve as the inaugural Command Sergeant Major, Joint Staff, Joint Force Headquarters.  CSM O’Brien was selected for her current position, as the State Command Sergeant Major on 1 July 2007.  She is the first female in California to serve as a Command Sergeant Major in a nominative position, (General Officer Command), and the first female to serve as the State Command Sergeant Major, California Army National Guard. 

In addition to her full-time position, Sergeant Major O’Brien has served four Adjutants General as a Quality Guard Advisor and Facilitator.  She is a certified in Strategic Management from the American Management Association, certified as Department of Defense Management Consultant, and an Army Communities of Excellence/Malcolm Baldridge Examiner.  During her two years as a Performance Examiner, she was also a writer for California’s submission, which won the Bronze Division in 1997 and Silver Division & Most Improved State in 1998.  She traveled to Europe with the Army Communities of Excellence Examination Team and was the first Non-commissioned Officer to lead an Examination Team.

She is a 1992 Distinguished Graduate of the U. S. Army Sergeants Major Academy, graduating at the top 2% of her class, a graduate of the U. S.  Marine Corps Staff Non-Commissioned Officer’s Academy, and numerous other Department of Defense, Army, Air Force, and Marine Corps schools and academies.  Her awards and decorations include the Meritorious Service Medal (5^th Award), the Army Commendation Medal (4^th Award), the Army Achievement Medal (3^rd Award), Army Good Conduct Medal (7^th Award), Marine Corps Good Conduct Medal (2^nd Award), and various other Marine Corps, Massachusetts, Louisiana and California National Guard awards and decorations.