Mindfulness in the parenting relationship has been proposed to help both parents and children better regulate stress, though this has not yet been shown at the physiological level. In this study, we tested relations between maternal mindfulness in parenting and both mothers’ and their infants’ hypothalamic-pituitary-adrenal (HPA) axis activity during a dyadic stressor 3 months later. Participants were 73 mother–infant dyads from a larger longitudinal study. At 3 months postpartum, mothers completed self-report measures of general dispositional mindfulness and parenting-specific mindfulness, as well as stressful life events. At 6 months postpartum, mother–infant dyads completed the Still Face task. Four saliva samples were collected from each dyad member for cortisol assay to index the HPA axis response. Hierarchical linear modeling of cortisol trajectories revealed a main effect of maternal parenting-specific mindfulness (mindful parenting), but not general dispositional mindfulness, on mothers’ cortisol; mothers with higher mindful parenting showed steeper cortisol recovery slopes. In addition, maternal mindful parenting moderated the effect of life stress on later mother and infant cortisol. In the context of high life stress, maternal mindful parenting predicted lower infant cortisol levels, but more extended maternal cortisol elevations. Implications for a biobehavioral model of mindful parenting are discussed.