This article presents theoretical arguments and supporting empirical evidence suggesting that attachment experiences in early life may be important in the later development of self-regulation and conscientious behavior. Analyses of data from the National Institute of Child Health and Human Development (NICHD) Study of Early Child Care and Youth Development (NICHD Early Child Care Research Network, 2005; N = 1,149) were conducted to test the association between attachment, measured at 15 and 36 months, and 3 measures of self-regulation (social self-control rated by teachers, task persistence as measured by observers in a series of lab tasks, and a continuous performance test) between Grades 1 and 5. Mediational analyses were also conducted to test whether self-regulation mediates the effect of attachment on children's engagement with learning in the classroom, as measured by direct observation. The results confirmed the hypothesis that attachment would be related to later self-regulation, but only for social self-control, and attentional impulsivity, not task persistence. Furthermore, social self-control at Grade 1 mediated the effect of attachment (at both 15 and 36 months) on school engagement at Grade 5, even when Grade 1 school engagement was statistically controlled. The discussion focuses on the potential importance of early attachment experiences for the development and maintenance of conscientiousness across the lifespan.