Key Points Question Was the COVID-19 pandemic associated with changes in mental health–related utilization for individuals with severe mental illness in the United States? Findings In this cohort study of more than 650 000 Medicare beneficiaries with schizophrenia or bipolar I disorders, there were statistically significant decreases in mental health–related outpatient, emergency department, and inpatient use as well as medication fills, particularly early in the pandemic, despite widespread use of telemedicine. Statistically significant differentially greater decreases in outpatient utilization were observed among individuals who were Black, had dual Medicare-Medicaid eligibility, or had disability. Meaning In this study, individuals with severe mental illness experienced substantial disruptions in care during the pandemic, and these disruptions were greater among disadvantaged populations., This cohort study examines changes in mental health care during the pandemic and the use of telemedicine in outpatient care among Medicare beneficiaries with severe mental illness., Importance Little is known about changes in care for individuals with severe mental illness during the COVID-19 pandemic. Objective To examine changes in mental health care during the pandemic and the use of telemedicine in outpatient care among Medicare beneficiaries with severe mental illness. Design, Setting, and Participants This population-based cohort study included Medicare beneficiaries (age ≥18 years) diagnosed with schizophrenia and schizophrenia-related disorders or bipolar I disorder. Care patterns during January to September 2020 for a cohort defined in 2019 were compared with those during January to September 2019 for a cohort defined in 2018. Exposures Start of COVID-19 pandemic in the United States, defined as week 12 of 2020. Main Outcomes and Measures Use of mental health–related outpatient visits, emergency department visits, inpatient care, and oral prescription fills for antipsychotics and mood stabilizers during 4-week intervals. Multivariable logistic regression analyses examined whether the pandemic was associated with differential changes in outpatient care across patient characteristics. Results The 2019 cohort of 686 214 individuals included 389 245 (53.8%) women, 114 073 (15.8%) Black and 526 301 (72.8%) White individuals, and 477 353 individuals (66.0%) younger than 65 years; the 2020 cohort of 723 045 individuals included 367 140 (53.5%) women, 106 699 (15.6%) Black and 497 885 (72.6%) White individuals, and 442 645 individuals (64.5%) younger than 65 years. Compared with 2019, there were large decreases during the pandemic’s first month (calendar weeks 12-15) in individuals with outpatient visits (265 169 [36.7%] vs 200 590 [29.2%]; 20.3% decrease), with antipsychotic and mood stabilizer medication prescription fills (216 468 [29.9%] vs 163 796 [23.9%]; 20.3% decrease), with emergency department visits (12 383 [1.7%] vs 8503 [1.2%]; 27.7% decrease), and with hospital admissions (11 564 [1.6%] vs 7912 [1.2%]; 27.9% decrease). By weeks 32 to 35 of 2020, utilization rebounded but remained lower than in 2019, ranging from a relative decrease of 2.5% (outpatient visits) to 12.9% (admissions). During the full pandemic period (weeks 12-39) in 2020, 1 556 403 of 2 743 553 outpatient visits (56.7%) were provided via telemedicine. In multivariable analyses, outpatient visit use during weeks 12 to 25 of 2020 was lower among those with disability (odds ratio, 0.95; 95% CI, 0.93-0.96), and during weeks 26 to 39 of 2020, it was lower among Black vs non-Hispanic White individuals (OR, 0.97; 95% CI, 0.95-0.99) and those with dual Medicaid eligibility (OR, 0.96; 95% CI, 0.95-0.98). Conclusions and Relevance In this cohort study, despite greater use of telemedicine, individuals with severe mental illness experienced large disruptions in care early in the pandemic. These narrowed but persisted through September 2020. Disruptions were greater for several disadvantaged populations.