This study identified mechanisms through which child–mother attachment security at 36 months was associated with mother- and teacher-reported friendship quality at 3rd grade. Data from a subsample of 1,071 children (536 boys) participating in the National Institute of Child Health and Human Development Study of Early Child Care and Youth Development were used. Separate structural equation models were tested for mother and teacher reports of peer functioning. For both models, the total indirect effect between attachment security and friendship quality was significant. Tests of specific indirect effects indicated that attachment security was associated with friendship quality via greater mother–child affective mutuality and better language ability at 54 months and fewer hostile attributions (teacher model only) and greater peer competence at first grade. The findings highlight interpersonal and intrapersonal mechanisms of attachment–friend linkages.