People experiencing homelessness (PEH) face a disproportionately high prevalence of adverse mental health outcomes compared with the non-homeless population and are known to utilize primary healthcare services less frequently while seeking help in emergency care facilities. Given that primary health services are more efficient and cost-saving, services with a focus on mental health that are co-designed with the participation of users can tackle this problem. Hence, we aimed to synthesize the current evidence of such interventions to assess and summarize the characteristics and effectiveness of co-designed primary mental healthcare services geared towards adult PEH. Out of a total of 10,428 identified records, four articles were found to be eligible to be included in this review. Our findings show that co-designed interventions positively impacted PEH's mental health and housing situation or reduced hospital and emergency department admissions and increased primary care utilization. Therefore, co-designed mental health interventions appear a promising way of providing PEH with continued access to primary mental healthcare. However, as co-designed mental health interventions for PEH can improve overall mental health, quality of life, housing, and acute service utilization, more research is needed.