Background: COVID-19 virus was reported to be transmitted through breastfeeding (BF), creating the need for rapidly available, standardized information and training for health personnel and the community about BF as an urgent action to reduce misinformation and unethical promotion of breast milk substitutes (BMS). In Mexico, a massive open online course (MOOC) was designed and implemented to protect, promote and support BF in emergency contexts. Methods: MOOC registration consisted of collection of quantitative data regarding participant characteristics, MOOC coverage, scores achieved and completion rates. A multiple linear regression analysis was performed to relate the absolute and relative skills earned in the MOOC to participant characteristics. In addition, factors associated with completion rate and dissemination of information from the MOOC were analyzed using multiple logistic regression and presented as odds ratios. Results: During a period of 19 months, 52,426 participants across the country, including health personnel and general population, entered the Cursos en Linea del Instituto Mexicano del Seguro Social (CLIMSS) platform, of which 50.5% completed the MOOC. The level of participation was maintained from January 2021 to early 2022 when the perception of the risk of getting sick from COVID-19 and quarantining decreased. In adjusted analysis, completion rate was associated with being older or belonging to a health institution; furthermore, residing in the north of the country doubled the odds of completing the MOOC (odds ratio 2.24; 95% confidence interval 1.95–2.56). Conclusions: A MOOC can be a useful training strategy to disseminate information, especially in emergencies where physical distancing is important and reaching the largest possible population is required.