Background <p>The purpose of this study was to examine the feasibility and fidelity of an 8-week teacher-guided active play intervention in an early childcare education setting. The secondary aim of this study was to examine the effects of the intervention on physical activity (PA) levels, time-on-task (TOT), and explore how teacher habit formation (automaticity) influenced implementation fidelity.</p> Method <p>Two preschool centers were randomly assigned to the intervention (<i>n</i> = 27 children, 3.91 ± 0.53&#xa0;years) or the control group (<i>n</i> = 25 children, 3.69 ± 0.81&#xa0;years). Teachers in the intervention group delivered 15-min indoor and outdoor active play opportunities for 8&#xa0;weeks. Retention measures occurred 14&#xa0;weeks later. Weekly checklists and informal interviews were conducted to assess intervention fidelity. Likert-style surveys were completed by teachers each week to assess automaticity. Wrist-worn accelerometers assessed PA during the school day. A modified time sampling technique was implemented for TOT observations.</p> Results <p>Teacher reports indicated the intervention was relatively easy to implement. The intervention group participated in significantly greater amounts of indoor PA from baseline to post-intervention (<i>F</i> (1,40) = 13.59, <i>p</i> &lt; 0.001, <i>η</i>_p² = 0.254) and from baseline to retention (<i>F</i> (1,33) = 16.84, <i>p</i> &lt; 0.001, <i>η</i>_p² = 0.994) when compared to the control group. Active play opportunities did not significantly impact TOT.</p> Conclusion <p>This study indicates that this type of intervention is feasible, particularly for indoor active play opportunities, and automaticity impacts implementation fidelity. Results of this study highlight the need to help decrease teacher barriers and increase active play during the preschool day.</p>