Introduction: Mental health interventions disseminated via, or accessed using, digital technologies are an innovative new treatment modality for managing co-morbid depression and substance use disorder. The present systematic review assessed the current state of this literature. Methods: A search of the Cochrane Library, Embase, Pubmed, PsycInfo and Scopus databases identified six eligible studies (N participants = 862), utilising quasi-experimental or randomised controlled designs. Reporting quality was evaluated and Hedges’ g effect sizes (with 95% confidence intervals and p-values) were calculated to determine treatment effectiveness. Process outcomes (e.g. treatment satisfaction, attrition rates) were also examined. Results: Quality ratings demonstrated high internal validity, although external validity was low. Effect size data revealed medium to large and short-term improvements in severity of depression and substance use symptoms in addition to global improvement in social, occupational and psychological functioning. Longer-term treatment effectiveness could not be established, due to the limited available data. Preliminary findings suggest that there was high client satisfaction, therapeutic alliance and client engagement. Discussion: Mobile phone devices and the Internet can help to increase access to care for those with mental health co-morbidity. Large-scale and longitudinal research is, however, needed before digital mental healthcare becomes standard practice. This includes establishing critical therapeutic factors including optimum levels of assistance from clinicians.