The COVID-19 pandemic completely changed daily lives and psychotherapy clinical operations. Treatment engagement with serious mental illness (SMI) populations is inherently challenging. COVID-19 may pose more challenges to treatment engagement with SMI populations due to the rapid change to telehealth and exacerbation of symptoms. SMI patients may be less likely to convert to telehealth or stay engaged in treatment compared to non-SMI patients during COVID-19. Nevertheless, treatment engagement via teletherapy is vital to supporting the vulnerable SMI populations during this stressful pandemic. In this study, we compared teletherapy conversion and utilization rates between SMI and general populations in an outpatient psychiatric clinic (n = 816) during COVID-19. We found no differences in telehealth conversion for patients with SMI (52%) and non-SMI (48%), suggesting equivalent adoption of teletherapy among SMI and non-SMI populations. Beyond conversion, we found that the SMI group had a significantly greater number of teletherapy visits compared to non-SMI patients, indicating that SMI group utilized teletherapy regularly after conversion. There were no group differences in new patients beginning therapy via telehealth, which shows that the SMI group was not more deterred to seek help. Our findings inform the feasibility of telehealth for serious mental illness populations during the pandemic.