Dr. Carrejo began his career with Kaiser as a Drug Education Coordinator (DEC) at Kaiser Richmond, and was soon given the additional responsibility of Kaiser Oakland Medical Center. He was then promoted to Clinical Operations Manager where he had oversight of the drug use management efforts and the ambulatory clinic pharmacists. In June of 1999 Ambrose was promoted to Drug Use Manager for Northern California. In this role he provided oversight and leadership for the Drug Education Coordinators of Northern California. In June of 2006, Dr. Carrejo was promoted to Assistant Director of Pharmaceutical Contracting. Finally in May, 2008 he was promoted to National Pharmaceutical Contracting Leader, where he was responsible for the contracting of all pharmaceutical purchases for the entire Kaiser Permanente program.