Mobile health units can improve access to preventive health services, especially for medically underserved populations. However, there is little published experience of mobile health units being used to expand access to COVID-19 vaccination. In concert with local public health departments and community members, we implemented a mobile COVID-19 health unit and deployed it to 12 predominantly low-income and racial/ethnic minority communities in Massachusetts. We describe the success and challenges of this innovative program in expanding access to COVID-19 vaccination. (Am J Public Health. 2022;112(11):1556-1559. https://doi.org/10.2105/AJPH.2022.307021).