This study examined potential processes of change and mechanisms of action in an 8-week manualized mindfulness intervention for adolescents with attention-deficit/hyperactivity disorder (ADHD) and their parents. Five families (five adolescents aged 13–18 and seven parents) participated in semi-structured interviews about their lived experiences of mindfulness 1 to 3 months after the intervention. Thematic analysis guided by a phenomenological approach revealed several potential underlying mechanisms of action contributing to improved peer and family relationship quality after mindfulness training. Themes were consistent among parents and adolescents. Participant descriptions were indicative of enhanced present-focused awareness and detached self-observation, contributing to improved self-monitoring and self-regulation of attention, behavior, and emotions. Participants reported becoming more adept at implementing adaptive emotion regulation strategies (e.g., cognitive reappraisal, problem-solving, and acceptance) and relying less on maladaptive emotion regulation strategies (e.g., rumination). Parents and adolescents described a parallel process of enhanced self-awareness and self-regulation that conjointly contributed to increased empathy, reduced emotional reactivity, improved communication, and reductions in the intensity and duration of conflicts. Furthermore, as individuals in the parent-child dyad became more adept at regulating their emotions, they mutually reinforced the emotion regulation skills of their social partner. A model of the co-regulatory process of change in parent-adolescent mindfulness training is proposed.