This paper describes a conceptual framework of maintenance of physical activity (PA) and its application to future intervention design. Evidence from systematic literature reviews and in-depth (N = 27) qualitative interviews with individuals with cancer were used to develop a conceptual framework of long-term physical activity behaviour. Determinants of long-term PA were listed and linked with domains of the Theoretical Domains Framework which in turn were linked to associated behaviour change techniques (BCTs) and finally to proposed mechanisms of action (MoA). The conceptual framework is presented within the context of non-modifiable contextual factors (such as demographic and material resources) and in the presence of learnt and adapted behavioural determinants of skills, competence and autonomous motivation that must be established as part of the initiation of physical activity behaviour. An inventory of 8 determinants of engagement in long-term PA after cancer was developed. Clusters of BCTs are presented along with proposed MoA which can be tested using mediation analysis in future trials. Understanding the processes of PA maintenance after cancer and presentation of implementable and testable intervention components and mechanisms of action to promote continued PA can inform future intervention development. This resource can act as a starting point for selection of intervention components for those developing future interventions. This will facilitate effective support of individuals affected by cancer to maintain PA for the long term.