The study of at-risk and radicalised youth has emerged as an area of academic interest. However, there are few models or theories to explain the process of youth radicalisation. This paper sets out a theoretical model for youth radicalisation drawing on Life-Course Theory and Situational Action Theory. The theoretical model is applied to three qualitative case studies. The case studies are drawn from a broader Australian study that used primary data to explore potential risk and contextual factors for youth referred to countering violent extremism (CVE) services before the age of 18 years. The findings of this study highlight how adverse developmental experiences (life-course factors) interact with situational conditions (environmental considerations) to shape the radicalisation of some youth. The detailed case studies presented in this paper highlight the need to adopt a developmental life-course lens to understand the nuanced nature of this pathway. The findings are considered in the context of the field of youth radicalisation research, and opportunities to incorporate life-course and situational factors to improve and progress practice-based responses to counter violent extremism with at-risk and radicalised youth.