The District Attorney's Office is the largest legal office in San Mateo County. Led by District Attorney Stephen M. Wagstaffe, the office employs 129 staff including skilled attorneys, investigators, program administrators, support staff and Victim Advocates. The office prosecutes felony and misdemeanor crimes committed in San Mateo County as well as traffic infractions, county ordinance violations, and juvenile cases. The office advises and conducts investigations for the Grand Jury. It also conducts investigations on theft, embezzlement and other cases pending trial. In 2020, the District Attorney's Office reviewed approximately 20,000 cases involving criminal offenses by adults and juveniles.

While best known for the prosecution of crime, the office serves the people of San Mateo County in additional ways. The Consumer and Environmental Protection Unit brings legal actions to prohibit unfair business practices and handles water pollution, hazardous waste and similar cases. The Victim Center provides crisis intervention, emergency assistance, resource & referral assistance, court assistance and assistance with filing an application for compensation of crime related expenses through the Victim Compensation & Government Claims Board.