Objective To evaluate concurrent and longitudinal associations between psychosocial functioning and physical activity in adolescent and young adult survivors of early childhood cancer. Methods Adolescent survivors of early childhood cancer (diagnosed before age four) participating in the Childhood Cancer Survivor Study completed the Coping Health and Illness Profile–Adolescent Edition (CHIP‐AE; n = 303; mean age at survey: 17.6 years). A subset of these survivors (n = 248) completed a follow‐up survey an average of 6.0 years later (range: 4‐10). Logistic regression identified associations between psychosocial functioning in adolescence and physical activity levels in adolescence and young adulthood. Results Survivors reported low physical activity as adolescents (46.1% scored below CHIP‐AE cut‐point) and young adults (40.8% below Centers for Disease Control guidelines). Poor physical activity during adolescence was associated with female sex (OR = 2.06, 95% CI, 1.18‐3.68), parents with less than a college education (OR = 1.91, 95% CI, 1.11‐3.32), previous treatment with cranial radiation (OR = 3.35, 95% CI, 1.69‐6.88), TV time (OR = 1.77, 95% CI, 1.00‐3.14), and limitations of activity due to health or mobility restrictions (OR = 8.28, 95% CI, 2.87‐30.34). Poor diet (OR = 1.84, 95% CI, 1.05‐3.26) and low self‐esteem (OR = 1.80, 95% CI, 0.99‐3.31) during adolescence were associated with lower odds of meeting Centers for Disease Control physical activity guidelines in young adulthood. Conclusion These findings provide targets for future interventional studies to improve physical activity in this high‐risk population.