The field of research impact assessment (RIA) has seen remarkable growth over the past three decades. Increasing numbers of RIA frameworks have been developed and applied by research funders and new technologies can capture some research impacts automatically. However, RIAs are too different to draw comparable conclusions about what type of methods, data or processes are best suited to assess research impacts of different kinds, or how funders should most efficiently implement RIAs. To usher in the next era of RIA and mature the field, future RIA methodologies should become more transparent, standardized and easily implementable. Key to these efforts is an improved understanding of how to practically implement and report on RIA at the funder-level. Our aim is to address this gap through two major contributions. First, we identify common items across existing best practice guidelines for RIA, creating a preliminary reporting checklist for standardized RIA reporting. Next, we systematically reviewed studies examining funders’ assessment of biomedical grant portfolios to examine how funders reported the results of their RIAs across the checklist, as well as the operational steps funders took to perform their RIA and the variation in how funders implemented the same RIA frameworks. We compare evidence on current RIA practices with the reporting checklist to identify good practice for RIA reporting, gaps in the evidence base for future research, and recommendations for future effective RIA.