Multisystemic therapy (MST) is an empirically validated, family and community‐based intervention for young people presenting with antisocial and offending behaviour. This qualitative study aimed to explore young people's experiences of MST and learn what had helped them to sustain positive outcomes over time. Semi‐structured interviews were conducted with eight young people at an average of 14 months after MST (range: 5–21 months). A constructivist version of grounded theory was employed to analyse the data, leading to the development of a model of sustained change in MST. Themes from the model included therapeutic alliance, increases in systemic awareness, recognizing responsibility, positive peer relationships, acknowledging and celebrating success, continued use of specific strategies (for example, worry boxes) and the identification and creation of a preferred future. This research presents an understanding of how change may be sustained after MST, highlighting systemic, developmental and individual factors in relation to this. Clinical implications and a proposed model of sustained change in MST are discussed. Practitioner points The therapeutic alliance was perceived by young people as central to the process of change and sustained change following MST Young people's contribution to sustaining therapeutic gains at follow up, alongside caregivers, highlighted the importance of actively engaging them in therapy Peer relationships were identified as relevant to sustaining change, particularly in relation to shared values and goals for the future