Flexible Assertive Community Treatment provides integrated and community-based support to clients with mental illness and challenges in social functioning. Music therapy has been incorporated into several Norwegian Flexible Assertive Community Treatment teams, but there is limited published research on its outcomes. The Flexible Assertive Community Treatment model is recovery oriented, and the significance of robust relationships with staff members is emphasised. To understand the processes and critical factors in the relationships between service users and their Music Therapist in Flexible Assertive Community Treatment, we addressed the following research question: What characterises the relationship and interaction between the Music Therapist and the service user in the context of Flexible Assertive Community Treatment? Six joint interviews with service users and their Music Therapist were conducted using a dyadic approach. Using reflexive thematic analysis, the characteristics of the music therapy relationship were categorised in three themes; (1) the humanistic, therapeutic relationship, (2) the friendship-like relationship and (3) the musical relationship. The relationship between the service user and the Music Therapist is essential to therapeutic outcome, and links to common factors in therapy. Building relationships with the Music Therapist and the Flexible Assertive Community Treatment team may be a vital step towards social participation for the service user and may be understood as a critical factor towards recovery.