This study tested bidirectional relations between infant high-intensity fear and fear regulation over 1.5 years, and maternal sensitivity and depressive symptoms as moderators. Participants were 1,292 mother-infant pairs prospectively assessed at three times when infants were 6, 15, and 24 months old. Infant high-intensity fear and fear regulation (avoidance, orienting to mother, attention regulation, and self-soothing) were observed during the Mask Task at each of these time points. Likewise, mothers reported their depressive symptoms, and their sensitivity was observed in a separate mother-child interaction task at each time point. Conditional multilevel growth models revealed that highly avoidant infants exhibited less initial high-intensity fear, but faster growth in high-intensity fear over time. Furthermore, highly avoidant infants exhibited more concurrent high-intensity fear when their mothers demonstrated low sensitivity. Unexpectedly, when their mothers were highly depressed, infants who used more attention regulation demonstrated more rapid growth of high-intensity fear over time. Finally, when their mothers were not depressed, infants exhibiting more high-intensity fear oriented more to their mothers concurrently. When their mothers reported high depressive symptoms over time, infants with more high-intensity fear initially oriented less to their mothers but more rapidly increased orienting to their mothers over time. Findings reveal the interplay between infant and maternal factors over time in development of infant high-intensity fear. In particular, maternal sensitivity protected highly fearful infants by rapidly reducing fear reactivity despite infant avoidance. In contrast, high maternal depressive symptoms introduced both immediate and enduring risks for infant fear development. Clinical implications are discussed.