Background: Patients with pre-existing severe mental disorders are significantly less likely to receive guideline-recommended cancer treatment and seems to have a significantly lower rate of cancer survival compared to patients with cancer without mental disorders. Aim: To perform a systematic review on barriers at patient-, provider- and system-levels in cancer trajectories of patients with pre-existing severe mental disorders. Method: A systematic review was performed following the PRISMA guidelines (PROSPERO ID: CRD42022316020). Results: Nine eligible studies were identified. Barriers at patient-level included lack of self-care and ability to recognize physical symptoms and signs. Provider-level barriers included stigma from health care professionals on mental disorders, whereas system-level barriers included fragmented health care and consequences of this. Conclusion: This systematic review found that barriers at patient-, provider- and system-levels exist in cancer trajectories for patients with severe mental disorders, causing disparities in cancer care. Further research is needed to improve cancer trajectories for patients with severe mental disorder.