The San Mateo County Sheriff’s Office Investigations Bureau provides thorough and effective investigative and specialized services to the public and San Mateo County criminal justice agencies to ensure safer communities and provide the foundation for investigation and successful prosecution of criminals while promoting countywide and area-wide collaborative partnerships. The San Mateo County Sheriff's Office services Half Moon Bay, Millbrae, San Carlos, Portola Valley, the unincorporated areas of Redwood City, Menlo Park, and East Palo Alto. Sam Trans buses, kiosks, vehicles, and facilities and Caltrain rail equipment, stations, and facilities are also policed by the Sheriff's Office.

The Cold Case Unit investigates unsolved dormant homicides, sexual assault cases, and missing person cases through reexamining evidence using modern forensic technology (e.g. - DNA or CODIS) and investigative techniques. Their investigations entail reviewing old case files, re-interviewing witnesses and families, and preparing cases for prosecution. Key duties include identifying new leads, maintaining chain of custody, and collaborating with forensic experts to build a new case from old evidence.