Adolescents attending Child & Adolescent Mental Health Services (CAMHS) requiring ongoing care are transferred to adult services (AMHS) at eighteen. Many young people with service needs are not being referred, or are refusing referral to AMHS. This study explored these issues from a social identity change perspective. Transcripts of interviews conducted with young people (n = 11), their parents (n = 5) and child (n = 11) and adult (n = 8) psychiatrists were thematically analysed. Transition to AMHS confirmed an illness identity. Young people adopting this identity saw continued service engagement as identity‐congruent. Disengagement was attributed to failure to adopt an illness identity or to an emerging adult identity associated with greater independence. Fractious professional relationships hindered transition and delayed the formation of a therapeutic alliance with AMHS staff. Disengagement post‐transfer was linked to incompatibility between the AMHS service remit and specific illness identities. This study demonstrates how an intersection between identities shapes service engagement and disengagement.