BackgroundEvaluations of primary healthcare co-located welfare advice services have been methodologically limited.AimsTo examine the impact and cost-consequences of co-located benefits and debt advice on mental health and service use.MethodProspective, controlled quasi-experimental study in eight intervention and nine comparator sites across North Thames. Changes in the proportion meeting criteria for common mental disorder (CMD, 12-item General Health Questionnaire); well-being scores (Shortened Warwick and Edinburgh Mental Well-being Scale), 3-month GP consultation rate and financial strain were measured alongside funding costs and financial gains.ResultsRelative to controls, CMD reduced among women (ratio of odds ratios (rOR) = 0.37, 95% CI 0.20–0.70) and Black advice recipients (rOR = 0.09, 95% CI 0.03–0.28). Individuals whose advice resulted in positive outcomes demonstrated improved well-being scores (β coefficient 1.29, 95% CI 0.25–2.32). Reductions in financial strain (rOR = 0.42, 95% CI 0.23–0.77) but no changes in 3-month consultation rate were found. Per capita, advice recipients received £15 per £1 of funder investment.ConclusionsCo-located welfare advice improves short-term mental health and well-being, reduces financial strain and generates considerable financial returns.