The vast generation of waste in the construction industry can be minimized by integrating circular economy (CE) principles and business models to reduce the circularity gap. Modular construction (MC) has been determined to be the best method for integrating CE principles toward more sustainability and energy efficiency. However, there is a lack of understanding about key determinant factors (KDFs) for MC-CE integration among clients and practitioners. This paper addresses this research gap by identifying KDFs using a systematic review method. Twenty-five KDFs were identified, after a rigorous synthesis of literature focusing on CE principles in the MC domain. The top five KDFs for MC-CE integration were: prompt completion of the MC project design, timely client commitment and consideration, level of understanding of the project leaders, degree of commitment toward collaborative working, and extent of knowledge/experience of the relevant contractors. Furthermore, an interdependency conceptual model is proposed, where all the KDFs are divided into people, technology, and process categories to facilitate clear guidelines for practical effectiveness. Finally, since digital technologies (DT) are essential to driving the CE and MC, a DT framework has been developed to drive increased MC-CE adoption, that will aid stakeholders in driving sustainability, resource efficiency, and waste reduction in the MC industry. This study provides MC practitioners and academics with a valuable guide for implementing CE principles in MC projects. The identified KDFs also led to a stakeholder checklist register that provides a foundation for further empirical research.