In Bangladesh and Viet Nam between 2009–2014, Alive & Thrive (A&T) worked to improve breastfeeding practices through intensified interpersonal counseling (IPC), mass media (MM), and community mobilization (CM) intervention components delivered at scale in the context of policy advocacy (PA). In Bangladesh, IPC was delivered through a large non-governmental health program; in Viet Nam, it was integrated into government health facilities. This study evaluated the population-level impact of intensified IPC, MM, CM, and PA compared to standard nutrition counseling with non-intensive MM, CM, and PA on breastfeeding practices in these two countries. These findings show that at scale interventions combining intensive IPC with MM, CM, and PA had greater positive impacts on breastfeeding practices in Bangladesh and Viet Nam than standard counseling with less intensive MM, CM, and PA. It is believed to be the first study to document implementation and impacts of breastfeeding promotion at scale using rigorous evaluation designs. Strategies to design and deliver similar programs at scale could improve breastfeeding practices in other contexts.