BACKGROUND: Outbreaks of SARS-CoV-2 in long-term care facilities (LTCFs) cause significant morbidity and mortality. Mapping viral transmission within and between facilities by combining genomic sequencing with epidemiologic investigations enables targeting infection-control interventions. METHODS: We conducted weekly surveillance of residents and staff in LTCFs in Santa Clara County, California, with ≥1 confirmed COVID-19 case between March and July 2020. Positive samples were referred for whole-genome sequencing. Epidemiological investigations and phylogenetic analyses of the largest outbreaks (>30 cases) were carried out in 6 LTCFs (Facilities A through F). RESULTS: Among the 61 LTCFs in the county, 41 had ≥1 confirmed case during the study period, triggering weekly SARS-CoV-2 testing. The 6 largest outbreaks accounted for 60% of cases and 90% of deaths in LTCFs, although the bed capacity of these facilities represents only 11% of the LTCF beds in the county. Phylogenetic analysis of 196 whole-genome sequences recovered from those facilities showed that each outbreak was monophyletic, with staff and residents sharing a common viral lineage. Outbreak investigations revealed that infected staff members often worked at multiple facilities, and in 1 instance, a staff member infected while working in 1 facility was the likely index case in another. CONCLUSIONS: We detected a pattern of rapid and sustained transmission after a single introduction of SARS-CoV-2 in 6 large LTCF outbreaks, with staff playing a key role in transmission within and between facilities. Infection control, testing, and occupational policies to reduce exposure and transmission risk for staff are essential components to keeping facility residents safe.