An inverse social gradient in early childhood overweight has been consistently described in high-income countries; however, less is known about the role of migration status. We aimed to study the social patterning of overweight in preschool children according to the mother’s socio-economic and migration background. For 9,250 children of the French ELFE birth cohort with body mass index collected at 3.5 years, we used nested logistic regression to investigate the association of overweight status in children (IOTF definition) with maternal educational level, occupation, household income and migration status. Overall, 8.3% (95%CI: [7.7–9.0]) of children were classified with overweight. The probability of overweight was increased for children from immigrant and descendants of immigrant mothers (OR 2.22 [95%CI 1.75–2.78] and 1.35 [1.04–2.78]) versus non-immigrant mothers. The highest probability of overweight was also observed in children whose mothers had low education, were unemployed or students, or from households in the lowest income quintile. Our findings confirm that socio-economic disadvantage and migration status are risk factors for childhood overweight. However, the social patterning of overweight did not apply uniformly to all variables. These new and comprehensive insights should inform future public health interventions aimed at tackling social inequalities in childhood overweight.