One in five older adults experience symptoms of depression and anxiety. Digital mental health interventions are promising in their ability to provide researchers, mental health professionals, clinicians, and patients with personalised tools for assessing their behaviour and seeking consultation, treatment, and peer support. This systematic review looks at existing randomised controlled trial studies on digital mental health interventions for older adults. Four factors have been found that contributed to the success of digital mental health interventions: (1) ease of use; (2) opportunities for social interactions; (3) having human support; and (4) having the digital mental health interventions tailored to the participants' needs. The findings also resulted in methodological considerations for future randomised controlled trials on digital mental health interventions: (1) having a healthy control group and an intervention group with clinical diagnoses of mental illness; (2) collecting data on the support given throughout the duration of the interventions; (3) obtaining qualitative and quantitative data to measure the success of the interventions; and (4) conducting follow-up interviews and surveys up to 1 year post-intervention to determine the long-term outcomes. The factors that were identified in this systematic review can provide future digital mental health interventions researchers, health professionals, clinicians, and patients with the tools to design, develop, and use successful interventions for older users.