The high prevalence of psychiatric disorders among resettled refugees necessitates identification of factors that reduce the risk of mental illness. In this 22-year longitudinal cohort study, we assessed whether the length of asylum-decision waiting periods is associated with resettled refugees' risk of being diagnosed with a psychiatric disorder. We used full-population data from the Danish Civil Registration System to establish a cohort of 46 104 refugees resettled in Denmark during 1995-2016. Hazard ratios (HRs) for first-time psychiatric hospital contact (ICD-10) after residence permit issuance across varying lengths of asylum-decision waiting periods were estimated by cross-linkage with the Danish National Patient Register. Long asylum-decision waiting periods were associated with an increased risk of psychiatric disorders. Compared with refugees who waited 0-6 months for their asylum decision, the HRs of any psychiatric diagnosis were 1.22 [95% confidence interval (CI): 1.12-1.33] for those who waited 13-24 months and 1.46 (95% CI: 1.27-1.69) for those who waited 25-71 months. Associations varied across diagnoses and length of follow-up: whereas the risk of nervous disorders increased with longer asylum-decision waiting periods in the follow-ups of 0-2.9, 3-5.9 and 6-11.9 years, the risk of psychotic disorders was associated with longer asylum-decision procedures only in the 0-2.9-year follow-up. Resettled refugees who waited longer than 1 year for an asylum decision face an increased risk of psychiatric disorders. Host countries should consider that long asylum-decision waiting periods could lead to mental illness among refugees. © The Author(s) 2019. Published by Oxford University Press on behalf of the International Epidemiological Association.