COVID-19 outbreak has promoted many public health measures in the general population. However, its impact on a vulnerable population with severe mental illness (SMI) is less addressed. Aim of this study was to determine the impact of COVID -19 to patients with SMI and identify its relation with their COVID-19 knowledge. A cross-sectional telephonic survey among 132 patients with SMI who were clinically stable before the COVID-19 pandemic was conducted. A 23 item interview proforma comprising of self-reported knowledge related to COVID-19 by patients and their illness and treatment status from their caregivers. Eleven patients were completely not aware of the ongoing COVID-19 pandemic. Three fourth of patients were not worried about getting COVID-19 and lacks adequate knowledge to identify symptoms. Two-third of patients lacked adequate knowledge of precautionary measures against COVID-19. One out of five patients lacked knowledge of the mode of transmission and stopped their psychiatric treatment. Thirty percent showed features of relapse of symptoms during this lockdown period. In multivariate regression analysis, patients from lower socioeconomic status, low literacy levels, with inadequate social support showed less knowledge related to COVID-19. Mental health services which target this vulnerable population during early disaster reduce the burden to the community.