The question of how to best measure family processes so that longitudinal experiences within the family are accurately captured has become an important issue for family scholars. Using the Fragile Families and Child Wellbeing Study (N = 2,158), this article focuses on the association between trajectories of perceived supportiveness from biological fathers and mothers' mental health problems 5 years after a birth. The relationship status between mothers and biological fathers is significantly related to her perceptions of his supportiveness, with married mothers reporting the highest levels of supportiveness followed by mothers in cohabiting unions, romantic non-coresidential unions, and, finally, mothers not in a romantic relationship. Controlling for both time-varying and time-invariant maternal and relationship characteristics, a positive slope of perceived supportiveness from biological fathers is associated with fewer subsequent mental health problems 5 years after the birth. The discussion calls attention to alternate modeling strategies for longitudinal family experiences. (Contains 3 tables, 1 figure, and 4 notes.)