Promoting exclusive breastfeeding (EBF) is a highly feasible and cost-effective means of improving child health. Regulating the marketing of breastmilk substitutes is critical to protecting EBF. In 1981, the World Health Assembly adopted the World Health Organization International Code of Marketing of Breastmilk Substitutes (the Code), prohibiting the unethical advertising and promotion of breastmilk substitutes. This comparative study aimed to (a) explore the relationships among Code enforcement and legislation, infant formula sales, and EBF in India, Vietnam, and China; (b) identify best practices for Code operationalization; and (c) identify pathways by which Code implementation may influence EBF. We conducted secondary descriptive analysis of available national-level data and seven high level key informant interviews. Findings indicate that the implementation of the Code is a necessary but insufficient step alone to improve breastfeeding outcomes. Other enabling factors, such as adequate maternity leave, training on breastfeeding for health professionals, health systems strengthening through the Baby Friendly Hospital Initiative, and breastfeeding counselling for mothers, are needed. Several infant formula industry strategies with strong conflict of interest were identified as harmful to EBF. Transitioning breastfeeding programmes from donor-led to government-owned is essential for long-term sustainability of Code implementation and enforcement. We conclude that the relationships among the Code, infant formula sales, and EBF in India, Vietnam, and China are dependent on countries' engagement with implementation strategies and the presence of other enabling factors.