We report the results of a quasi-experimental evaluation of a mystery shopper intervention in Zacatecas and Guadalupe, Mexico. Underage youth attempted to purchase beer at 50 Modelorama stores and 32 Oxxo stores (intervention groups), and at 19 comparison convenience stores in March, July, and August 2018. After each attempt, intervention store operators were informed if a sale was made. Modelorama operators also received training and were warned that repeated sales to minors could jeopardize their franchise. Average sales rates to minors were 63.8% at Modeloramas, 86.5% at Oxxo stores, and 98.2% at comparison stores. The findings suggest that mystery shopper interventions with training, feedback to store operators, and sanctions after repeated sales to underage youth may reduce sales to minors in low- and middle-income countries.