In India, globalization has caused a nutrition transition from home-cooked foods to processed sugary snacks and drinks, contributing to increased early childhood caries (ECC). This mixed-methods study describes risk factors for ECC and associations with undernutrition in low-income communities in Mumbai. Interviews with mothers of 959 children, ages six-months through six-years, addressed maternal-child nutrition and oral health, and children received dental exams and anthropometric assessments. Focus groups with community health workers and mothers explored experiences and perceptions of oral health, nutrition, and ECC. Descriptive and logistic regression analyses of quantitative data, and content analysis of qualitative data were performed. Eighty percent of children lived 5 min from a junk-food store, over 50% consumed junk-food and sugary tea daily, 50% experienced ECC, 19% had severe deep tooth decay, 27% experienced mouth pain, and 56% experienced chronic and/or acute malnutrition. In children ages 3-6, each additional tooth with deep decay was associated with increased odds of undernutrition (Odds Ratio [OR] 1.10, Confidence Interval [CI] 1.02-1.21). Focus groups identified the junk-food environment, busy family life, and limited dental care as contributors to ECC. Policy interventions include limits on junk-food marketing and incorporating oral health services and counseling on junk-food/sugary drinks into maternal-child health programs.