The incidence of developmental delay and early intervention (EI) service utilization is not well documented among unauthorized Mexican immigrants, a vulnerable population. Individual interviews were conducted in Spanish with Mexican born women receiving maternal health care. Children 12–60 months of age were screened for developmental delay using the Ages and Stages Questionnaire. 12 % (n = 8) of children assessed (n = 65) were at risk for developmental delay. Of those at risk 38 % (n = 3) participated in EI. An additional 26 % of the children (n = 17) qualified for further monitoring, and of those 59 % (n = 10) received EI. Women with low health literacy had more than four times the odds of having a child with risk of developmental delay (aOR 4.4; 95 % CI 1.3–15.4). Developmental delay was associated with low maternal health literacy in unauthorized Mexican immigrants; however, rates of self-reported EI use in this population are higher than those seen nationally.