Background: Patients with severe mental illness (SMI) may be at increased risk for COVID-19-related severe morbidity and mortality. There is limited research on the vaccination rates against COVID-19 in patients with SMI. Aims: The objective of the present study is to explore vaccination rates and co-relations in patients with SMI, attending community mental health services, namely the Mobile Mental Health Units (MMHUs) in rural Greece. Method: All treatment engaged patients with SMI (schizophrenia-spectrum or bipolar disorder) with two MMHUs (MMHU of Kefalonia, Zakynthos and Ithaca, [MMHU KZI] and MMHU of the prefectures of Ioannina and Thesprotia [MMHU I-T]) in rural Greece were enrolled prospectively over a six-month period. The MMHU I-T had adopted a more proactive approach to patients’ vaccination, by informing patients and caregivers for its benefits. Results: Data were analyzed for 197 patients with SMI. The overall vaccination rate was 68.5% and did not differ from the respective rates in the general population. There were no differences in vaccination rates among patients attending the two MMHUs, nor among patients with different diagnoses (schizophrenia spectrum disorders or bipolar disorder). Vaccination was not associated with gender, educational level, history of alcohol and substance abuse, illness duration, or number of previous hospitalizations, whereas the effect size of age was moderate. In more than half non-vaccinated patients the refusal to get vaccinated was associated with fears and concerns as well as false beliefs that are encountered in the general population. Conclusion: In the present sample of treatment-engaged rural patients vaccination rates against COVID-19 appear to be satisfactory. There were no differences in vaccination rates with regard to the interventions that were applied to enhance vaccination. It seems that other forms of intervention should be applied to reluctant patients to modify their attitudes toward vaccination.