PURPOSE: This review aimed to evaluate interventions led by peer counselors (PCs) for adults with common mental disorders in low- and middle-income countries (LMICs) along indices of overall effectiveness and key implementation outcomes (acceptability, feasibility, cost, fidelity, sustainability). METHODS: This review followed a mixed-methods systematic review design. MEDLINE/PubMed, Embase, PsycINFO, and Global Health databases were searched for PC-led interventions for adults in LMICs targeting depressive and/or anxiety disorders or PTSD. Quantitative data was narratively synthesized, and qualitative data was thematically synthesized separately. The results from the qualitative and quantitative syntheses were then combined in a cross-study synthesis. RESULTS: Twenty-four papers describing thirteen PC-led interventions were included for review. Narrative synthesis results indicated mixed effectiveness of PC-led interventions in reducing depressive, anxiety, PTSD symptoms and high PC competency. Thematic synthesis revealed five descriptive themes: (1) Preferred PC characteristics; (2) Incentives and motivation for PCs; (3) Barriers to PC-led intervention implementation; (4) Helpful supervision/training practices; and (5) Overall high acceptability of PC-led interventions. Cross-study synthesis revealed high acceptability, feasibility, and fidelity, but cost and sustainability outcomes were underreported in included papers. CONCLUSION: PC-led interventions seem to show initial promise in terms of effectiveness, acceptability, feasibility, cost, fidelity, and sustainability. Future research should focus on standardizing measurements of implementation outcomes to facilitate cross-study analysis. Additional empirical attention should be paid to underrepresented implementation outcomes (e.g., cost, sustainability). Finally, researchers should adopt a participatory approach that elevates the perspectives of PCs throughout all stages of the implementation process.