Background: Evidence links tummy time (ie, the time spent in awake prone positioning that is encouraged and supervised by an adult) to infants’ health outcomes such as gross motor and total development. However, the associations between tummy time and other movement and sleep behaviors as the child develops remain unknown. The aim of this study was to examine whether early introduction and practice of tummy time within the first 6 months of age were associated with active and outdoor playtime, screen time, and nocturnal sleep time of children when they were 12 and 24 months old. Methods: A longitudinal analysis was conducted using data extracted from an Australian trial. Using telephone surveys with mothers, demographic data were collected from third trimester of pregnancy and tummy time data were collected at 6 months of age. Data on playtime, screen time, and nocturnal sleep duration as dependent variables were collected at 12 and 24 months of age. Multiple logistic regression models were built to investigate the associations. Results: Children who started tummy time within 4 weeks of age were more likely to have >10 hours sleep at night at 12 months (adjusted odds ratio 1.54, 95% confidence interval, 1.08–2.19). They were more likely to have >3 hours per day of outdoor play and have <1 hour per day of screen time at 24 months. Children who practiced tummy time every day were more likely to have >2 hours per day of active play at 12 months and have <1 hour per day of screen time at 24 months. Conclusions: Starting tummy time earlier and frequently was associated with more favorable movement and sleep of young children at 12 and 24 months of age.