Background Maternal depression is associated with increased risk of psychiatric illness in offspring. While risk may relate to depressed mothers’ difficulties regulating emotions in the context of interacting with offspring, physiological indicators of emotion regulation have rarely been examined during mother–child interactions—and never among mother–adolescent dyads in which both mother and adolescent have histories of major depressive disorder (MDD). Methods We examined changes in high‐frequency heart rate variability (HF‐HRV), an indicator of parasympathetic (vagal) function that has been related to depression, stress, social engagement, and emotion regulation, in 46 mother–daughter dyads (23 in which both mother and daughter had an MDD history and 23 never‐depressed controls). Hierarchical linear models evaluated changes in HF‐HRV while mother–daughter dyads engaged in discussions about shared pleasant events and relationship conflicts. Results While control dyads displayed positive slopes (increases) in HF‐HRV during both discussions, MDD dyads displayed minimal change in HF‐HRV across discussions. Among controls, HF‐HRV slopes were positively correlated between mothers and daughters during the pleasant events’ discussion. In contrast, HF‐HRV slopes were negatively correlated between MDD mothers and daughters during both discussions. Conclusions Vagal responses observed in control mother–daughter dyads suggest a pattern of physiological synchrony and reciprocal positive social engagement, which may play a role in adolescent development of secure social attachments and healthy emotion regulation. In contrast, MDD mothers and daughters displayed diminished and discordant patterns of vagal responsiveness. More research is needed to understand the development and consequences of these patterns of parasympathetic responses among depressed mother–daughter dyads.