Background: There is a growing body of evidence showing the benefits to coaches and players in adopting a game-based pedagogical approach. Whilst the evidence in support of a game-based pedagogy continues to rise it is acknowledged that the complex art form of coaching is a uniquely personal one, where the coach may draw on previous first-hand experiences and traditional coaching practices regarding training methods rather than the use of current evidence-based best practice techniques. Purpose: The aim of this randomised control trial was to evaluate the impact of a coach development intervention (MASTER) on game-based coaching practices of football coaches. Methods: Four clubs were recruited, and 20 coaches were randomised to two groups, MASTER intervention (n = 10) and 10 usual practice (waitlist, n = 10); NSW Australia) which included 200 junior footballers (aged 8--16years). Intervention coaches participated in an 8-week multi-component intervention (which included a coach education workshop focused on positive game-based pedagogy, mentoring, peer evaluations and an online discussion forum) underpinned by positive coaching and game-based coaching practices. Pre- and post-intervention assessments occurred at baseline and 10 weeks. The primary aim was to investigate if the MASTER intervention could increase playing form (PF) and active learning time within training sessions. Three coaching sessions per coach were filmed at baseline and follow-up and assessed using the MASTER assessment tool. Secondary aims investigation included coach confidence and competence to coach (assessed by questionnaire), player game play and decision making (videoed during structured game play using a Game Performance Assessment Instrument), a range of player well-being measures including enjoyment, self-perception, and various motivations (questionnaire) and the parent's perception of their child's enjoyment was assessed by the completion of a questionnaire. Intervention effects were analysed using linear mixed models. Findings: Significant effects were found for the primary outcome which was the percentage of training time devoted to playing-form activities (22.63%; 95% CI 9.07--36.19; P = 0.002, d = 1.78). No significant effect was observed for ALT. Significant interventions effects were also observed for the secondary outcomes of coach perceptions of confidence and confidence; player game skills including defence, support and decision making, wellbeing, physical self-perceptions, enjoyment, learning and performance orientations and motivation; and parent's perception of child's enjoyment (P < 0.05). Conclusions: The MASTER programme was effective in improving game-based coaching practices of football coaches during training sessions, and in facilitating improvements in multiple coach and player outcomes.