Water Safety Plan (WSP) implementation has the potential to greatly improve, commonly very challenging and resource limited, small drinking water supplies. Although slower than in urban or high-income settings, the uptake of WSPs in low- and middle-income countries (LMICs) is accelerating. Understanding the factors which will make a WSP successful will further improve efficient uptake and assist with its long-term sustainability. Based on an extensive literature search using the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISM-A) methodology, 48 publications, including case studies and guidance documentation, formed the basis of this review. These were analysed using inductive and deductive coding methods to (i) identify the success factors applicable to WSP implementation in small drinking water supplies in LMICs and (ii) to investigate which factors are more or less critical depending on the geography and level of development of the implementing country. Key challenges identified during the review process were also noted. A comparison of these success factors was made with those identified from high-income and urban settings. The three most important success factors identified are the development of technical capacity, community engagement, and monitoring and verification. Factors specific to small drinking water supplies in LMICs include support from non-government organisations, integration into existing water sanitation and hygiene (WASH) programs, simplicity, and community engagement. Certain factors, such as adaptability, the use of guidance documentation, international collaboration, the role of pilot studies, knowledge sharing, and stakeholder involvement are applicable to all WSP settings. Due to the specific challenges faced by small drinking water systems and the limited number of original research publications on this topic, this study highlights the need for further data collection and research focused on success factors in these settings. It is anticipated that the consideration of the success factors identified in this study will assist implementers in improving the uptake and long-term sustainability of WSPs in small drinking water supplies in low- and middle-income settings.