The period after school represents an opportunity to engage children in physical activity (PA) programmes in schools. The purpose of this study was to evaluate the effect of an afterschool programme, delivered in schools, on children's participation in and attitudes to PA. Ten schools took part in this non-randomized controlled trial (five experimental and five control schools). Experimental schools participated in an after-school programme delivered by teachers and parents for 50 min per week. Outcome measures included device-measured PA, self-report youth PA behaviour and parental perceptions of the school environment. Measurements took place at baseline, and mean follow-up was at 10 weeks. Data were collected from 196 participants, with a mean age of 8.1 ± 0.8 years. Mean daily minutes of moderate-to-vigorous physical activity (MVPA) for the entire sample at baseline was 69.9 ± 23.3. While both groups presented a decline in MVPA levels, there was a significant difference in the change from baseline to follow-up (P = 0.043) of min/day MVPA between the control group (-13.2 ± 23.9) and the experimental group (-0.9 ± 25.2). This programme may help maintain MVPA levels, with children exposed to the intervention experiencing a significantly lower decline in MVPA than their control counterparts.