Background Family mindfulness‐based intervention (MBI) for child attention‐deficit/hyperactivity disorder (ADHD) targets child self‐control, parenting and parental mental health, but its effectiveness is still unclear. Methods MindChamp is a pre‐registered randomised controlled trial comparing an 8‐week family MBI (called ‘MYmind’) in addition to care‐as‐usual (CAU) (n = 55) with CAU‐only (n = 48). Children aged 8–16 years with remaining ADHD symptoms after CAU were enrolled together with a parent. Primary outcome was post‐treatment parent‐rated child self‐control deficits (BRIEF); post hoc, Reliable Change Indexes were explored. Secondary child outcomes included ADHD symptoms (parent/teacher‐rated Conners’ and SWAN; teacher‐rated BRIEF), other psychological symptoms (parent/teacher‐rated), well‐being (parent‐rated) and mindfulness (self‐rated). Secondary parent outcomes included self‐ratings of ADHD symptoms, other psychological symptoms, well‐being, self‐compassion and mindful parenting. Assessments were conducted at post‐treatment, 2‐ and 6‐month follow‐up. Results Relative to CAU‐only, MBI+CAU resulted in a small, statistically non‐significant post‐treatment improvement on the BRIEF (intention‐to‐treat: d = 0.27, p = .18; per protocol: d = 0.33, p = .11). Significantly more children showed reliable post‐treatment improvement following MBI+CAU versus CAU‐only (32% versus 11%, p < .05, Number‐Needed‐to‐Treat = 4.7). ADHD symptoms significantly reduced post‐treatment according to parent (Conners’ and SWAN) and teacher ratings (BRIEF) per protocol. Only parent‐rated hyperactivity impulsivity (SWAN) remained significantly reduced at 6‐month follow‐up. Post‐treatment group differences on other secondary child outcomes were consistently favour of MBI+CAU, but mostly non‐significant; no significant differences were found at follow‐ups. Regarding parent outcomes, significant post‐treatment improvements were found for their own ADHD symptoms, well‐being and mindful parenting. At follow‐ups, some significant effects remained (ADHD symptoms, mindful parenting), some additional significant effects appeared (other psychological symptoms, self‐compassion) and others disappeared/remained non‐significant. Conclusions Family MBI+CAU did not outperform CAU‐only in reducing child self‐control deficits on a group level but more children reliably improved. Effects on parents were larger and more durable. When CAU for ADHD is insufficient, family MBI could be a valuable addition.