Maternal depression is associated with a range of effects on child development, including difficult temperament. This longitudinal study investigated the impact of depressive symptoms (DS) that mothers experience after childbirth on infant negative affect (NA), as well as potential effects of infant NA on maternal DS, across the first year of life. In the study (N = 63), identical questionnaires (the Beck Depression Inventory II, and the Infant Behavior Questionnaire – Revised, Very Short Form) were administered at 2 weeks, 4, 6, and 9 months after birth. Using path analysis, we tested five different models that could explain the relationship between maternal DS and infant NA. The best-fitting model showed that the days immediately following childbirth represent an important time for the development of infant temperament as maternal mood impacts significantly on infant NA for at least 4 months after birth. This does not constitute a single sensitive period; a new predictive effect emerges around 4 months of age, suggesting cascading influences of maternal DS across the first 6 months of life. These results suggest a need for support should a mother experience DS, not only immediately after birth but throughout the early stages of parenting.