Importance: During the COVID-19 pandemic, the US federal government required that skilled nursing facilities (SNFs) close to visitors and eliminate communal activities. Although these policies were intended to protect residents, they may have had unintended negative effects. Objective: To assess health outcomes among SNFs with and without known COVID-19 cases. Design, Setting, and Participants: This retrospective observational study used US Medicare claims and Minimum Data Set 3.0 for January through November in each year beginning in 2018 and ending in 2020 including 15477 US SNFs with 2985864 resident-years. Exposures: January through November of calendar years 2018, 2019, and 2020. COVID-19 diagnoses were used to assign SNFs into 2 mutually exclusive groups with varying membership by month in 2020: active COVID-19 (≥1 COVID-19 diagnosis in the current or past month) or no-known COVID-19 (no observed diagnosis by that month). Main Outcomes and Measures: Monthly rates of mortality, hospitalization, emergency department (ED) visits, and monthly changes in activities of daily living (ADLs), body weight, and depressive symptoms. Each SNF in 2018 and 2019 served as its own control for 2020. Results: In 2018-2019, mean monthly mortality was 2.2%, hospitalization 3.0%, and ED visit rate 2.9% overall. In 2020, among active COVID-19 SNFs compared with their own 2018-2019 baseline, mortality increased by 1.60% (95% CI, 1.58% to 1.62%), hospitalizations decreased by 0.10% (95% CI, -0.12% to -0.09%), and ED visit rates decreased by 0.57% (95% CI, -0.59% to -0.55%). Among no-known COVID-19 SNFs, mortality decreased by 0.15% (95% CI, -0.16% to -0.13%), hospitalizations by 0.83% (95% CI, -0.85% to -0.81%), and ED visits by 0.79% (95% CI, -0.81% to -0.77%). All changes were statistically significant. In 2018-2019, across all SNFs, residents required assistance with an additional 0.89 ADLs between January and November, and lost 1.9 lb; 27.1% had worsened depressive symptoms. In 2020, residents in active COVID-19 SNFs required assistance with an additional 0.36 ADLs (95% CI, 0.34 to 0.38), lost 3.1 lb (95% CI, -3.2 to -3.0 lb) more weight, and were 4.4% (95% CI, 4.1% to 4.7%) more likely to have worsened depressive symptoms, all statistically significant changes. In 2020, residents in no-known COVID-19 SNFs had no significant change in ADLs (-0.06 [95% CI, -0.12 to 0.01]), but lost 1.8 lb (95% CI, -2.1 to -1.5 lb) more weight and were 3.2% more likely (95% CI, 2.3% to 4.1%) to have worsened depressive symptoms, both statistically significant changes. Conclusions and Relevance: Among skilled nursing facilities in the US during the first year of the COVID-19 pandemic and prior to the availability of COVID-19 vaccination, mortality and functional decline significantly increased at facilities with active COVID-19 cases compared with the prepandemic period, while a modest statistically significant decrease in mortality was observed at facilities that had never had a known COVID-19 case. Weight loss and depressive symptoms significantly increased in skilled nursing facilities in the first year of the pandemic, regardless of COVID-19 status.