Dispositional mindfulness has been shown to protect against affective symptoms in the general population. However, very little is known about whether and how these benefits may extend to a particularly high-risk period for affective distress—the postpartum. In this study, we tested within-person and between-person associations between maternal mindfulness and symptoms of anxiety and depression across the first 18 months postpartum. We further investigated whether mindfulness moderated the effect of life stress on mothers’ symptoms. Participants were 89 mothers from a larger longitudinal study on mother-infant stress regulation. Mothers completed self-report measures of dispositional mindfulness, life stress, anxiety, and depression at 3, 6, 12, and 18-months postpartum. Hierarchical linear modeling was used to assess the impact of mindfulness and life stress on both symptom trajectories and deviations from those trajectories over time. Absolute levels of maternal mindfulness predicted lower maternal depression symptoms at 18 months, and relative increases in mindfulness predicted concurrent decreases in both anxiety and depression symptoms over time. There was no evidence for moderated effects; rather, life stress related independently to anxiety and depression. Implications for understanding mindfulness as a dynamic construct and potential applications to improving postpartum mental health are discussed.