Although studies suggest that patients with diabetes with a serious mental illness (SMI) have poor diabetes outcomes, reports conflict regarding the quality of their diabetes care and level of glucose control. In an observational follow-up to our initial cross-sectional study, we compared glucose control (glycosylated hemoglobin [HbA1c]) between patients with diabetes with SMI versus those without SMI at two postbaseline assessments during an approximately 5-year period. Both groups continued to have glucose levels higher than what is considered good control and neither group demonstrated a significant change in mean HbA1c at the two follow-up time points. Those with SMI continued to have lower HbA1c levels than those without SMI even after adjusting for potential confounders. More effective strategies are needed to assist patients with diabetes to improve the management of their glucose levels.