Objectives: Although people with a migration background (MB) have more unmet mental health needs than the general population, patients with a MB are still underrepresented in mental health care services. Provider bias towards these patients has been evidenced repeatedly but its driving factors remain elusive. We assessed the moderating effect of the individual (e.g. age and ethnicity), interpersonal (e.g. healthcare provider trust), and organisational (e.g. perceived workload) factors on general practitioners (GPs) differential decision-making regarding diagnosis, treatment, and referral for a depressed patient with or without a MB. Design: An experimental study was carried out in which GPs were shown one of two video vignettes featuring adult male depressed patients, one with a MB and the other without. Belgian GPs (n = 797, response rate was 13%) had to decide on their diagnosis, treatment, and referral. Analysis of variance and logistic regression were used to analyse the effect of a MB, adding interaction terms for the explanatory variables. Results: Overall, we found that there were ethnic differences in GPs’ decisions regarding diagnosis and treatment recommendations. GPs perceived the symptoms of the patient with a MB as less severe (F = 7.68, p < 0.01) and demonstrated a reduced likelihood to prescribe a combination of medical and non-medical treatments (F = 11.55, p < 0.001). Those differences increased in accordance with the GP’s age and perceived workload; at an interpersonal level, we found that differences increased when the GP thought the patient was exaggerating his distress.