Background: Patients with mental health conditions (MHCs) face barriers in health care and often have undertreated comorbidities. The purpose of this dissertation was to evaluate the treatment and prognosis of patients with atrial fibrillation (AF) suffering from MHCs. Methods: The nationwide registry-based Finnish Anticoagulation in Atrial Fibrillation cohort covers all patients diagnosed with AF in Finland during 2004–2018 at any level of care. The use of oral anticoagulant (OAC) therapy and rhythm control therapies, as well as ischemic stroke, bleeding and mortality outcomes, were assessed in patients with and without depression, anxiety disorder, bipolar disorder, schizophrenia or any MHC. Results: In total, 239,222 patients diagnosed with incident AF in Finland between 2007 and 2018 were identified in this study, with a 19.9% prevalence of any MHC. Patients with any MHC were less likely to initiate OAC therapy than patients without MHCs (64.9% vs. 73.3%, p < 0.001). Patients with MHCs had similar adherence to non-vitamin K antagonist oral anticoagulants (NOACs) in the implementation phase of the therapy, but they discontinued NOAC therapy 16 % more often than patients without MHCs. Rhythm control therapies, including antiarrhythmic drugs, cardioversion and catheter ablation, were used less often for patients with MHCs compared to patients without MHCs. Crude rates of ischemic stroke, bleeding and mortality were all higher in patients with MHCs than in patients without MHCs. None of the MHC categories were independently associated with the risk of ischemic stroke, but any MHC, depression and schizophrenia were associated with higher mortality. Furthermore, any MHC, depression and anxiety disorders were associated with a higher risk of bleeding. The lower use of OAC therapy partly explained the higher crude mortality and ischemic stroke rates in patients with MHCs. Conclusions: Interventions are needed to improve stroke prevention in patients with AF and MHCs. Patients comorbid with MHCs have worse crude outcomes than patients without MHCs. MHCs are independently associated with higher mortality and bleeding risks but not with the risk of ischemic stroke in patients with AF.