Chronic insecurities that emerge from adverse experiences in early intimate partner relationships in adolescence and emerging adulthood can have profound impacts on mental health and well‐being. Less clear is the extent to which these experiences for parents impact subsequent relationships within and across generations. We examine the extent to which secure, dismissing, pre‐occupied, and fearful intimate partner relationships in adolescence and emerging adulthood, well before becoming a parent, are associated with next‐generation patterns of attachment between mothers and infant offspring. Data were drawn from a nested study of infant–mother attachment (n = 220) within the Australian Temperament Project Generation 3 Study (N = 1167, est. 1983). Intimate partner relationships in adolescence and young adulthood were assessed by self‐report at 23–24 years of age. Over a decade later, infant–mother attachment security was assessed at 12 months post‐partum. Young adult intimate partner relationships defined by high levels of fearful, pre‐occupied, and dismissing attachment styles were reported in 11%, 17%, and 38% of young mothers, respectively. Increases in fear of intimacy in relationships were associated with an increase in the odds, by around 50%, of infant–mother insecure attachments (vs secure; OR = 1.56, 95% CI = 1.07, 2.28) and disorganised attachments (vs organised; OR = 1.49, 95% CI = 1.00, 2.22). A mother's self‐reported history of fear of intimacy within young adult relationships predicts later insecure and disorganised mother–infant attachments. Guidance and greater support for young people navigating their earliest intimate relationships may not only prevent adverse relational experiences at the time but also on becoming a parent. Findings have relevance for family and infant mental health therapies. Translating these findings into supported conversations may help prevent infant–mother attachment difficulties, or later repair them, through validation of the lingering effects of early fear of intimacy and empowerment of parents to prevent next‐generation infant experiences of distrust.