Severe mental illness (SMI) is associated with increased risk for type 2 diabetes, partly due to adverse metabolic effects of antipsychotic medications. In public health care settings, annual screening rates are 30%. We measured adherence to national diabetes screening guidelines for patients taking antipsychotic medications. To estimate diabetes screening prevalence among patients with SMI within an integrated health care system, and to assess characteristics associated with lack of screening. Retrospective cohort study. Antipsychotic-treated adults with SMI. We excluded participants with known diabetes. Primary outcome was screening via fasting glucose test or hemoglobin A1c during a 1-year period. In 2014, 16,754 patients with SMI diagnoses were receiving antipsychotics. Seventy-four percent of these patients’ providers ordered diabetes screening tests that year, but only 55% (9247/16,754) received screening. When the observation time frame was extended to 2 years, 73% (12,250/16,754) were screened. Adjusting for sex and race/ethnicity, young adults (aged 18-29 years) were less likely to receive screening than older age groups [adjusted RR (aRR) 1.23-1.57, p < 0.0001]. Compared to whites, screening was more common for Asians (aRR 1.141, 95% CI 1.089-1.195, p < 0.0001), less common for blacks (aRR 0.946, 95% CI 0.898-0.997, p < 0.0375), and no different for Hispanics (aRR 1.030, 95% CI 0.988-1.074, p = 0.165). Smokers were less likely to be screened than non-smokers (aRR 0.93, 95% CI 0.89-0.97, p < 0.0008). Utilization of either mental health or primary care services increased the likelihood of screening. While almost three-fourths of adults with SMI taking antipsychotic medications received a lab order for diabetes screening, only 55% received screening within a 12-month period. Young adults and smokers were less likely to be screened, despite their disproportionate metabolic risk. Future studies should assess the barriers and facilitators with regard to diabetes screening in this vulnerable population at the patient, provider, and system levels.