Achieving universal health coverage is a key component of the Sustainable Development Goals, focusing on equitable access to quality health services and minimising financial hardship. While strategies often target the demand-side, supply-side barriers such as travel time and facility-level constraints are often overlooked. Accurately quantifying who is affected by these barriers and identifying their locations and specific barriers is critical to improving service delivery. This study examines these barriers in Mali, a country with significant health system challenges exacerbated by high fertility rates and political instability. Using the WHO's Health Resources and Services Availability Monitoring System, we conduct an analysis of the geographic accessibility of antiretroviral therapy (ART) services. Our aim is to estimate the number of people affected by supply-side barriers by using a geographic accessibility model that calculates travel time to facilities with ART services. The analysis applies a least-cost path algorithm to assess accessibility, where ART services are defined as accessible within 2 hours travel time. People within this range with available services have access while those outside are geographically constrained. For those within 2 hours but without ART access, we identified and quantified facility-specific barriers. The results show that nearly 2.7 million Malians do not have timely access to ART within 2 hours. For about 70%, distance is the main barrier, while the rest face facility-level issues such as the fact that the service is not being planned in the facility, lack of medical supplies and lack of training. This study offers important insights for targeted interventions to scale up ART provision and provides a scalable model for other health services and contexts.