Background Telemental health care has been rapidly adopted for maintaining services during the COVID-19 pandemic, and a substantial interest is now being devoted in its future role. Service planning and policy making for recovery from the pandemic and beyond should draw on both COVID-19 experiences and the substantial research evidence accumulated before this pandemic. Objective We aim to conduct an umbrella review of systematic reviews available on the literature and evidence-based guidance on telemental health, including both qualitative and quantitative literature. Methods Three databases were searched between January 2010 and August 2020 for systematic reviews meeting the predefined criteria. The retrieved reviews were independently screened, and those meeting the inclusion criteria were synthesized and assessed for risk of bias. Narrative synthesis was used to report these findings. Results In total, 19 systematic reviews met the inclusion criteria. A total of 15 reviews examined clinical effectiveness, 8 reported on the aspects of telemental health implementation, 10 reported on acceptability to service users and clinicians, 2 reported on cost-effectiveness, and 1 reported on guidance. Most reviews were assessed to be of low quality. The findings suggested that video-based communication could be as effective and acceptable as face-to-face formats, at least in the short term. Evidence on the extent of digital exclusion and how it can be overcome and that on some significant contexts, such as children and young people’s services and inpatient settings, was found to be lacking. Conclusions This umbrella review suggests that telemental health has the potential to be an effective and acceptable form of service delivery. However, we found limited evidence on the impact of its large-scale implementation across catchment areas. Combining previous evidence and COVID-19 experiences may allow realistic planning for the future implementation of telemental health.