Physical activity (PA) has been found to improve physical and mental health and aid recovery in those with serious mental illness (SMI). However, individuals with SMI conduct less PA than the general population but little is known about how people with SMI adopt PA and what is involved in their behaviour change processes. The aim of this study is to explore individual experiences of PA to elucidate the behaviour change processes of PA in people with SMI who are in recovery. Method: A hermeneutic phenomenological approach was undertaken. Eight active participants (4 male, 4 female) who were in recovery with either bipolar disorder or schizophrenia, were interviewed and their data thematically analysed. Findings: Four main themes emerged which identified behaviour change facilitators when initiating and maintaining engagement in PA. Three themes revealed how participants became more active: ‘Not ready to engage’; ‘Initial steps to engaging in PA’ and ‘Becoming more active’. Within these themes, a variety of findings emerged, including: an awareness of the body in existence, a PA enabling environment and feeling real and normal. The fourth main theme, was labelled ‘Doing PA’, this outlined the experienced acts of PA. The type of PA conducted had different beneficial outcomes on the perceived symptoms of SMI. Individuals developed related PA preferences, which motivated them to continue with those activities. Conclusions: Individuals with SMI could be encouraged to conduct more PA by supporting individually meaningful PA. Strategies are suggested which may help individuals to initially engage in PA, but also to continue engaging in PA by enhancing their PA experience.