Background Multimorbidity and frailty are often associated and Comprehensive Geriatric Assessment (CGA) is considered the gold standard of care for these patients. Aims This study aimed to evaluate the effect of outpatient Comprehensive Geriatric Assessment (CGA) on frailty in community-dwelling older people with multimorbidity and high health care utilization. Methods The Ambulatory Geriatric Assessment—Frailty Intervention Trial (AGe-FIT) was a randomized controlled trial (intervention group, n  = 208, control group n  = 174) with a follow-up period of 24 months. Frailty was a secondary outcome. Inclusion criteria were: age ≥ 75 years, ≥ 3 current diagnoses per ICD-10, and ≥ 3 inpatient admissions during 12 months prior to study inclusion. The intervention group received CGA-based care and tailored interventions by a multidisciplinary team in an Ambulatory Geriatric Unit, in addition to usual care. The control group received usual care. Frailty was measured with the Cardiovascular Health Study (CHS) criteria. At 24 months, frail and deceased participants were combined in the analysis. Results Ninety percent of the population were frail or pre-frail at baseline. After 24 months, there was a significant smaller proportion of frail and deceased ( p  = 0.002) and a significant higher proportion of pre-frail patients in the intervention group ( p  = 0.004). Mortality was high, 18% in the intervention group and 26% in the control group. Conclusion Outpatient CGA may delay the progression of frailty and may contribute to the improvement of frail patients in older persons with multimorbidity.