Mexican American children are disproportionately affected by obesity. Data on how the acculturation process influences diet and body weight among adolescents are limited. We used the data from the 1999-2004 NHANES, restricting to 2286 Mexican American children between 12 and 19 y old. Acculturation was measured by generation status and language preference. Diet was assessed using 24-h diet recall. Multiple linear, Tobit, logistic, and quantile regression models were used. We found, after adjusting for socio-demographic factors, health, dietary intake, physical activity, and sedentary behaviors compared to the first generation, second and third generations had greater odds of overweight and obesity. Adolescents in the second generation had higher BMI Z-scores than adolescents in the first and third generations. Both second and third generation adolescents consumed less fruit, whole fruit, vegetables, grains, and meats but more sweetened beverages, whole grains, saturated fat, sodium, oil, and energy from discretionary foods. Higher language acculturation was associated with poorer diet and greater body weight. Our findings suggest that Mexican American adolescents face challenges in terms of poorer diet and excessive weight gain associated with their immigration experience.