Objective: People with psychosis or bipolar disorder (severe and persistent mental illness ) are at high risk for poor psychiatric and chronic illness outcomes, which could be ameliorated through improved health care quality. This study assessed whether a telehealth, collaborative care program managed by psychiatric clinical pharmacists (SPMI Population Care) was associated with improved health care quality for adults with SPMI in a large California health system. Methods: This retrospective cohort study used electronic health record data to compare 968 program enrollees at 6 demonstration sites (Population Care) to 8,339 contemporaneous patients with SPMI at 6 non-program sites (Usual Care). SPMI diagnoses were based on ICD-10-CM diagnostic codes. Primary outcomes were optimal psychotropic medication adherence, guideline-recommended glycemic screening, annual psychiatrist visit, and emergency department use. Difference-in-difference analyses assessed change in outcomes from 12 months pre- to 12 months post-enrollment using overlap weighting with high dimensional propensity scores to balance participant characteristics across groups. Participant data were collected from January 1, 2020, to June 30, 2022. Results: From pre- to post-enrollment, Population Care was associated with greater achievement of psychotropic medication adherence and glycemic screening (+6 and +9 percentage points), but unexpectedly with a decrease in annual psychiatrist visits (−6 percentage points) and no significant change in emergency department use, relative to Usual Care. More than 75% of Population Care participants attended an intake and ≥ 1 follow-up visits. Participants with psychosis (26% of sample) had similar results as those with bipolar disorder. Conclusions: Clinical pharmacist–led telehealth collaborative care has potential to improve psychopharmacologic treatment adherence and recommended disease preventive screening for people with psychosis or bipolar disorder. J Clin Psychiatry 2024;85(1):23m14917 Author affiliations are listed at the end of this article.