Although effortful control (EC), a regulatory aspect of temperament, is associated with a wide range of developmental outcomes, knowledge about EC promoters is scarce. This study explored whether secure attachment promoted the development of EC from preschool to school age in a community sample of 903 Norwegian children. EC was measured using the parent-reported Children’s Behavior Questionnaire at four (T1) and six (T2) years of age, and attachment was measured using the Manchester Child Attachment Story Task at T1. Previous research has indicated that a child’s gender and socioeconomic status are possible covariates of EC; hence, these factors were included in the analyses. Despite considerable rank-order stability in EC, secure attachment contributed to an increase in EC. Furthermore, gender moderated the effect of attachment: secure attachment promoted EC in boys only. These findings emphasize preschool boys’ need for emotional security to facilitate effortful capacities in their transition to school.