BACKGROUND: Relatively little is known about the prognostic value of comorbid mental disorders in alcohol use disorder (AUD) treatment for older adults (OA). AIMS: This article aimed to investigate 1) the impact of current unipolar mood and anxiety disorders in AUD treatment success in OA, 2) the timing of this putative comorbidity impact over six months, and 3) the role of treatment length in comorbidity effects. METHODS: We analyzed baseline and one-, three-, and six-month follow-up data from the international multicenter RCT "ELDERLY-Study" (baseline n = 693, median age: 64.0 years) using mixed effects regression models. In adults aged 60+ with DSM-5 AUD "ELDERLY" compared outpatient motivational enhancement therapy (MET, four sessions) with outpatient MET plus community reinforcement approach for seniors (MET & CRA-S; up to 12 sessions). Aiming for abstinence or minimal alcohol use (AU), both conditions included CBT-elements. We assessed AU with Form 90, and mental disorders with the Mini International Neuropsychiatric Interview (M.I.N.I.). RESULTS: Mood-related disorders were associated with more drinks per day at baseline and greater reductions in drinks per day at one and six months (main effect mood disorder: Coef. 2.1, 95% CI 0.6-3.6; one month interaction effect: Coef. -1.9, 95% CI -3.3- -0.5; six months interaction effect: Coef. -2.1, 95% CI -3.5 - -0.6). These results were replicated within MET & CRA-S but not within MET. CONCLUSION: Comorbid mental disorders had modest effects on short-term outpatient treatment outcomes. OA with AUD and unipolar mood-related disorders may profit from short interventions based on motivational interviewing and CBT-elements. ClinicalTrials.gov:NCT02084173.