To better understand the long-term implications of coparenting quality for adult child outcomes, we examined the associations between coparenting quality in the family of origin (Generation 1; G1), and attachment avoidance and anxiety and perceived relationship functioning of new parents (Generation 2; G2) using a dyadic approach. Dual-earner families expecting their first child (n = 182) were followed across the transition to parenthood and assessed at the third trimester of pregnancy (3T) and 9 months after childbirth (9M). At 3T, parents reported on the coparenting quality in their families of origin, and attachment avoidance and anxiety. At 9M, the participants reported their perceptions of couple relationship functioning—dyadic adjustment and negative interaction. We found that at 9M, G1’s coparenting quality predicted not only G2’s own perceptions but also G2’s partners’ perceptions of relationship functioning. Further, mediational analyses showed that parents’ G1 coparenting quality was associated with higher G2 self- and partner-perceived dyadic adjustment and lower G2 self-perceived negative interaction through G2 parents’ lower attachment anxiety and avoidance. G1 coparenting quality was negatively associated with G2 partner-perceived negative interaction through G2 parents’ lower attachment anxiety. Our findings suggest that coparenting relationships have long-term implications for human development even into adulthood.