Children of depressed mothers have impaired cognitive, behavioral, and health outcomes from infancy through adulthood, and are especially at risk when maternal depression persists over multiple years. But there are several important limitations to our current descriptive knowledge about maternal depression, especially depression among unmarried mothers. Data from the Fragile Families and Child Wellbeing Study, a recent cohort of children born in urban areas to mostly unmarried parents (N = 4,366), was used to examine the prevalence and correlates of maternal depression when children were about 1, 3, 5, and 9 years old. Results show that, at any given survey wave, between 16% and 21% of mothers reported depression. Nearly two-fifths (38%) of mothers reported depression at least once during the eight-year period, and 7% reported persistent depression (depression at three or four of the four survey waves). Employment status, relationship status, and fathers’ depression were among the sociodemographic characteristics most robustly associated with both stability and change in maternal depression. Given the important social consequences of maternal depression, not least of which is impaired wellbeing among children of depressed mothers, prevention and treatment of maternal depression should be an imperative for researchers, clinicians, and policymakers alike.