This study sheds light on the role of informal street markets in local Mexican food environments. Less is known about the availability, accessibility, and affordability of fresh food offered in these markets and their contribution to urban health in cities of the Global South in which high degrees of informality exist. We examined the impacts of informal markets on food environments in Mexico City, Guadalajara, Monterrey, Queretaro, and San Luis Potosí. We conclude that informal markets significantly improve availability, access, and affordability of fresh food, especially in the central region of Mexico and for vulnerable low-income groups on the urban periphery. However, they play a very limited role in car-centric cities of the industrial north such as Monterrey. This study aims to support planning decisions that target the improvement of access to a healthy diet in informal settings.