This study aimed to investigate the influence of case management and its corresponding computer-assisted assessment system on the quality improvement of dementia care. This observational study enrolled 2029 patients and their caregivers at Changhua Christian Hospital in Taiwan. Physicians who made the diagnosis of dementia would introduce the patient and caregiver dyad to the case manager-centered collaborative care team after obtaining agreement. The achievement rates of 11 quality indicators (QIs) comprising timely diagnostic evaluations, regular screens of cognition and neuropsychiatric symptoms, caregiver support, and proper medication prescriptions were counted. Different timeframes (≤4 months, 4 months-1 year, 1-2 years, 2-3 years, or ≥3 years) from diagnosis of dementia to collaborative care intervention were compared. A significantly higher attainment rate was achieved for patients with earlier entry into the collaborative team model, including QIs about timely diagnosis and regular screening, and caregiver support. The QIs regarding dementia medication prescriptions and documentation of the risk of antipsychotics remained similar regardless of the time of entry into the model. The completion rates of QIs also improved after the information system was launched. Physician-case manager co-management in the setting of a collaborative care model with a computer-assisted assessment system helps improve QI achievement for dementia care.