Classroom teachers are increasingly called upon to help increase pupils’ physical activity (PA). Grounded in Guskey’s model of teacher change, this study was part of an intervention that provided classroom teachers with training for implementing classroom-based PA (CBPA). The study examined teachers’ attempts to implement CBPA and focused on their self-reported practices (number of CBPAs implemented and management routines used) as well as their perceptions of this process. Participants (N = 13) were teachers from a U.S. primary school. Data collection included self-reported number of CBPAs implemented (in the year prior to, and the year of, the intervention), self-assessment of management routines used, semi-structured interviews, self-reflection cards, and field notes from classroom observations. Descriptive statistics were calculated and group mean comparison tests were performed for the number of CBPAs implemented and the use of management routines. Qualitative data analysis involved constant comparison and analytic induction techniques, along with several trustworthiness techniques. Teachers implemented a significantly greater number of CBPAs on most days during the study compared to the year before (M = 2.55, SD = 1.36, Cohen’s d = 1.88) and indicated a high level of consistency relative to the use of management strategies taught. Four themes were generated from the qualitative data: (a) positive teacher perceptions of CBPA; (b) perceived positive pupil outcomes and responses; (c) common management routines and common challenges; and (d) teacher support and valuable feedback for the intervention. Findings suggest that sustained professional development can provide classroom teachers with the knowledge and skills needed to regularly implement CBPAs. Also, findings indicate the usefulness of Guskey’s model of teacher change in studying classroom teachers’ involvement in PA promotion programmes and provide valuable insights that can inform similar future projects.