Prior research has established the importance of co-parenting for child outcomes, yet little is known about how co-parenting influences parents themselves. The current study expands on the prior literature by examining an important aspect of co-parenting, perceived parenting agreement, and exploring the longitudinal association of perceived parenting agreement with new parents’ depression, positive affect, and relationship satisfaction during the transition to parenthood. Using a dyadic approach, results indicated there were significant actor effects of parenting agreement on both mothers’ and fathers’ mental health outcomes, such that greater agreement predicted better subsequent mental health. Perceived parenting agreement was also significantly associated with subsequent relationship satisfaction for mothers only, such that greater perceived agreement by both the mother (actor effect) and the father (partner effect) predicted greater subsequent maternal relationship satisfaction. Our results suggest perceptions of parenting agreement are important for new parents’ mental health and maternal relationship satisfaction during the transition to parenthood.