This study examines whether the strength of the parent‐therapist alliance, measured during the task of alliance building with parents alone, predicted the extent of parents’ attachment‐promoting behaviour in the subsequent conjoint parent‐adolescent attachment task in the context of attachment‐based family therapy (ABFT). Nineteen suicidal and depressed adolescents and their parents received 12 weeks of ABFT. The parent‐therapist alliance and parents’ attachment‐promoting behaviour were each measured observationally by two separate and independent groups of raters. The findings show that the strength of the parent‐therapist alliance predicted parents’ subsequent attachment‐promoting behaviour. However, parents’ attachment‐promoting behaviour did not predict the post‐treatment outcomes. Clinical and methodological implications are discussed. Practitioner Points Good parent-therapist alliances are linked to parents being empathic, curious and non‐defensive during emotional conversations with their suicidal and depressed adolescents. Parental attachment‐promoting behaviour, including attunement, empathy, and active listening, may not always be sufficient curative factors. Once the parent‐adolescent attachment relationship improves, parents may support their adolescent as they cope with school and peer conflict.