Effective implementation strategies are important for take-up of programs in schools. However, to date, few implementation strategies have been co-designed with teachers and support staff (including principals) in Australia. The aim of this study was to iteratively co-design multiple implementation strategies to enhance the delivery of mental health prevention program, PAX Good Behaviour Game, in New South Wales primary schools. The secondary aim was to evaluate the acceptability of the implementation strategies from the perspective of school staff. Twenty-nine educational staff (including principals) informed the co-design of the implementation strategies across three phases. Phase 1 involved a rapid review of the literature and stakeholder meetings to agree upon potential evidence-based strategies. Phase 2 involved focus group discussions with educational staff to co-design implementation strategies. Phase 3 involved semi-structured interviews with school staff to assess strategy acceptability after implementation at 6-months post-baseline. Data were analysed using deductive, framework analysis. The final co-designed intervention included nine implementation strategies accessible through a toolkit delivered to the school’s leadership team. These strategies were deemed acceptable in school settings that experienced periods of both face-to-face and remote learning due to the changing COVID-19 situation in 2021. This paper contributes to the implementation literature by transparently reporting how educational staff-informed implementation strategies were iteratively co-designed. This will provide a roadmap for other researchers to co-design implementation strategies to further support the delivery of evidence-based prevention programs in schools.