While there is evidence that parks support pediatric health, there have been no national studies looking at both physical and mental health. We assessed whether the presence of a neighborhood park is associated with pediatric physical or mental health across the U.S. using a nationally representative cross-sectional random sample of American children ages 0–17. Caregivers reported on the park presence in their child's neighborhood and the child's physical activity, screen-time, sleep, weight, and diagnosis of anxiety, depression, or attention deficit hyperactivity disorder (ADHD). Covariates included child and family sociodemographics and, for 29 states, neighborhood urbanicity. Caregivers reported on 49,146 children (mean age 9.4 years; 49% female). There were 11,791 (24%) children living in neighborhoods lacking a park; children in non-urban locations (aOR 2.19, 95% CI 1.40–1.67) or below the federal poverty level (aOR = 1.48, 95%CI 1.38–1.58) had higher odds of lacking a park. Irrespective of sociodemographics, children lacking parks were more likely to be physically inactive (aOR1.36, 95% CI 1.24, 1.48), have excessive screen-time (aOR = 1.19, 95% CI 1.14, 1.25), or obtain inadequate sleep (aOR = 1.23, 95% CI 1.18, 1.29). Children without parks were more likely obese (aOR = 1.32, 95% CI 1.21, 1.43), overweight (aOR 1.25, 95%CI 1.17, 1.33), or diagnosed with ADHD (aOR 1.20, 95% CI 1.12, 1.29), but not more anxious or depressed (aOR = 1.04, 95%CI 0.97, 1.11). Associations between parks and pediatric physical and mental health suggests that the provision of neighborhood parks could represent a low-cost childhood health intervention.