Mental health encompasses the ability to cope with important stresses of life and to realize one’s abilities in the community, and the COVID-19 pandemic represented a very stressful event for people with mental illnesses. Our aim was to assess mental well-being in people living with different mental disorders, comparing results obtained in 2016 with those observed after the COVID-19 pandemic. Ninety-six participants were assessed using the Mental Health Continuum Short Form and classified as “flourishing,” “moderately mentally healthy,” and “languishing.” Overall, a significant increase in the prevalence of “flourishing” and “moderately mentally healthy” subjects and a reduction of “languishing” subjects (p = 0.003) were observed. However, a significant improvement in well-being (p = 0.005) was observed only in the schizophrenia spectrum disorder group. Moreover, only subjects that never contracted SARS-CoV-2 showed a significant (p = 0.019) increase in positive well-being states. Lockdowns may have led caregivers to spend more time with the participants, also increasing treatment adherence, resulting in an improvement of overall well-being in several participants.