Childhood obesity is being considered a global health epidemic, and one of the countries mostly affected by it is Mexico. The aim of this study was to assess the perceptions of low-income mothers with regard to their child’s weight status and physical activity and their beliefs about healthy and high-density foods. A total of 813 mothers attending a vaccination centre at three primary care clinics in three different regions of Mexico, and their infants ranging from 5 to 24 months old, participated in the study. Anthropometrical measurements and interviews were conducted at the clinic. The child’s average age was 12.7 months. Forty-three percent of mothers underestimate their child’s weight status; this percentage jumps to 83% when we refer to mothers of overweight and obese children. Mothers with overweight and obese children, having a monthly income higher than 600 dollars, less than 6 years of education, and having migrated to their current state of residence are more likely to underestimate their child’s weight. A high percentage of the mothers wish their children were less active, and they do not consider that carbonated and non-carbonated sweetened drinks and high fat snacks might be dangerous to their child’s health. In conclusion, low-income, Mexican mothers highly underestimate their infant’s weight status and have higher risks of fostering an obesogenic environment.