Nutrition plays an important role in proper physical and cognitive functioning. However, there is limited evidence on the relationship between overall diet, cognition, and academic success in children, particularly among low-income and diverse groups. The objective of this study was to examine the relationships between healthful versus less healthful food group intake, cognitive performance, and academic achievement in a diverse sample of schoolchildren. 868 urban schoolchildren (age 8 to 10 years) participated in the study. Intake of healthful (fruits, vegetables, unsweetened beverages) and less healthful (sweet and salty snacks, sugar-sweetened beverages) food groups was determined via a food frequency questionnaire. Digit Span and Stroop test scores were used to assess cognitive performance. Academic achievement was assessed via standardized test scores. Multiple Poisson and multiple linear regression were used to test the associations between diet and cognitive scores. Multiple ordered logistic regression was used to assess the associations between diet and academic achievement. Potential confounders (age, sex, body mass index (BMI) z-score, race/ethnicity, English language learner status, individualized education plan enrollment, physical activity, and parent education level) were tested for inclusion in all models. The sample included 868 children (56.7% girls; 33.2% non-Hispanic white, 26.2% Hispanic, 17.1% multiracial/other, 8.3% non-Hispanic black; 40.5% overweight/obese). The most frequently consumed foods were fruits and sweet snacks (1.9 and 1.6 servings per day, respectively). There were no statistically significant associations between diet and cognitive test scores. Greater intake of less healthful food groups (sweet snacks, salty snacks, and sweetened beverages) was associated with lower math (OR = 0.91, CI [0.84, 0.98], p = 0.014) and English standardized test scores (OR = 0.87, CI [0.80, 0.94, p = 0.001). Greater intake of sweet snacks and fruits was associated with lower English scores (OR = 0.72, 95% CI [0.59, 0.88] p = 0.001; and OR = 0.75, 95% CI [0.72, 0.94] p = 0.003, respectively). Consumption of less healthful food groups was associated with poorer academic achievement. Further research may shed light on unexpected associations between fruit consumption and achievement. Policies targeting multiple dietary components may positively influence child academic achievement and development.