Girls Active is a physical activity programme, delivered in UK secondary schools, with the aim of increasing moderate-to-vigorous physical activity (MVPA) in girls aged 11-14 years. This study presents the process evaluation as part of a 14-month cluster randomised controlled trial designed to evaluate the effectiveness of the Girls Active programme and which showed no difference in the primary outcome (MVPA at 14 months) between intervention and control arms.Quantitative and qualitative data were collected from intervention schools over the course of the 14 month trial. Feedback forms and attendance records were completed at the end of all teacher and peer leader training and review days. At 7- and 14-months, semi-structured interviews were conducted with the lead Girls Active teacher in all intervention schools (n = 10) and staff from the intervention provider (n = 4) and hub school (n = 1). At 14 months, separate focus groups with peer leaders (n = 8 schools), girls who participated in the evaluation component of the trial (n = 8 schools), and a sample of boys (n = 6 schools) were conducted. All participants in the intervention schools were asked to complete an exit survey at 14 months. Teachers (intervention and control) completed a school environment questionnaire at baseline, 7- and 14-months.The Girls Active programme, i.e., the training and resources, appeared to be well received by teachers and pupils. Factors that may have contributed to the lack of effectiveness include: some initial uncertainty by teachers as to what to do following the initial training, a predominant focus on support activities (e.g., gathering opinions) rather than actual physical activity provision, and school-level constraints that impeded implementation.Girls Active and what it was trying to achieve was valued by schools. The programme could be improved by providing greater guidance to teachers throughout, the setting of timelines, and providing formal training to peer leaders.ISRCTN, ISRCTN10688342 . Registered 12 January 2015.