Background: Recess is an opportunity for children to engage in daily physical activity. The aim of this study was to investigate the 12-month effects of a playground intervention on children’s moderate-to-vigorous (MVPA) and vigorous physical activity (VPA) during morning and lunchtime recess. Methods: Four hundred and seventy children (232 boys, 238 girls) from 26 elementary schools participated in the study. Fifteen schools redesigned the playground environment using playground markings and physical structures. Eleven schools served as socioeconomic matched controls. Physical activity levels were quantified using heart rate and accelerometry at baseline, 6 months, and 12 months post-intervention. A 3-level (time, pupil, and school) multilevel analysis was used to determine the effects of the intervention across time on MVPA and VPA. Results: Positive yet nonsignificant intervention effects were found for MVPA and VPA during morning and lunchtime recess. Intervention children were more active during recess than control children. Interactions revealed that the intervention effect was stronger at 6 months than 12 months post-intervention. Conclusions: A playground markings and physical structures intervention had a positive effect on intervention children’s morning and lunchtime MVPA and VPA when assessed using heart rate and accelerometry, but this effect is strongest 6-months post-intervention and decreased between 6 months and 12 months.