Plastic pollution has reached an undeniable level of global impact. The transition towards circular systems that rethink the design and use of plastic products and eliminate plastic waste leakage is critical to protect human health and the environment. Urgency around interventions has reached every level of governance worldwide. However, gaps remain in measuring circularity to inform interventions and evaluating intervention impacts over time in order to adapt accordingly. It is important to collect this data to inform decision-making, particularly in a manner that is comparable across contexts and in a way that the data and methods are accessible and relatable. As the United Nations (UN) negotiations for a legally binding agreement on plastic pollution move towards National Action Plans (NAP) and localized interventions to meet global mandates, it is critical for local stakeholders who know their context best to retain tools for monitoring, evaluation, and decision-making. The objectives of this dissertation are focused on defining circularity through a critical review and application of the Circularity Assessment Protocol (CAP) to holistically quantify circularity for plastic packaging in multiple contexts. These contexts include an analysis of CAP results across six cities in five countries as part of the Urban Ocean initiative, the application of CAP to set a baseline and develop interventions in a Small Island Developing State (SIDS), and the use of CAP to inform and measure the impact of solutions at the state-level when deployed in several cities in Florida, United States. Findings include the absence of and need for harmonized circularity definitions and metrics that are globally relevant and comparable, the outsized benefits of upstream interventions to optimize local circularity, and the unique challenges and opportunities of cities and small island developing states in preventing plastic pollution. These findings have informed campaigns, policy, infrastructure, and other interventions in the study locations. This research finds that CAP can be an effective, replicable, and scalable tool to quantify circularity for plastic packaging in various contexts and for different use cases globally. It demonstrates the value of, and need for, this work as stakeholders worldwide enter the next era of plastic pollution prevention.