The stress process perspective suggests that romantic relationship transitions can be stressors that impair mental health. Research on romantic relationships and mental health has ignored one common stressor, on-again/off-again relationships, or churning. Using five waves of data from the Fragile Families and Child Wellbeing Study (N = 3,176), we examine associations between relationship churning and mothers’ mental health. We find that mothers experiencing relationship churning have worse mental health than mothers in stably together relationships, net of characteristics associated with selection into relationship instability; these associations persist over four years. Mothers experiencing relationship churning have similar mental health as their counterparts who experience union dissolution (with or without repartnering). Current relationship status and quality explain some of the differences between churning and stably together mothers. Findings emphasize attending to multiple types of family stressors—even stressors and instability in ongoing relationships—and the micro-level ecological factors that shape mental health.