It has been shown that following exposure to mild perinatal adversity, children have greater susceptibility to both the negative and positive aspects of their subsequent environment. In a large population-based cohort study (N=1,776), we investigated whether mild perinatal adversity moderated the association between maternal harsh parenting and children's hair cortisol levels, a biomarker of chronic stress. Mild perinatal adversity was defined as late preterm birth (gestational age at birth of 34-37 weeks, 6 days) or small for gestational age (birth weight between the 2.5th and 10th percentile for full term gestational age). Harsh parenting was assessed by maternal self-report at 3 years. Children's hair cortisol concentrations were measured from hair samples collected at age 6. There were no significant bivariate associations between mild perinatal adversities and harsh parenting and hair cortisol. However, mild perinatal adversities moderated the association between maternal harsh parenting and hair cortisol levels. Children with mild perinatal adversity had lower cortisol levels if parented more harshly and higher cortisol levels in the absence of harsh parenting than children who did not experience mild perinatal adversity. These results provide further evidence that mild perinatal adversity is a potential marker of differential susceptibility to environmental influences.