To examine the longitudinal associations of objectively measured physical activity and modified organized sport participation with executive functions and psychological health in preschoolers. One hundred and eighty-five preschool children, mean age 4.2 ± 7.68; (years:months), 34% girls were followed for one year. Physical activity was measured using accelerometery, examining light, moderate, vigorous, moderate-to-vigorous, and total physical activity. Parents reported children’s participation in modified organized sport. Direct assessment of children’s executive functions (working memory, inhibition and shifting) and educator-reported psychosocial difficulties were also collected. Associations were examined using linear regression adjusting for covariates, baseline developmental outcomes and preschool clustering. Vigorous physical activity at baseline was positively associated with children’s shifting performance (b = 0.245; 95% CI: 0.006, 0.485, p =.045) at follow-up, while the association for moderate-to-vigorous physical activity approached significance (b = 0.119; 95% CI: −0.001, 0.239, p =.051). Children not participating in modified organized sport at baseline demonstrated better inhibition scores 12-months later compared to sports participants (Mdiff 0.06; CI: 0.00, 0.13, p =.046). Increasing time spent in higher intensity physical activity among preschool children may be a viable target for supporting their later cognitive development, although there was no clear benefit of early participation in modified organized sport.