Attention Deficit Hyperactivity Disorder (ADHD) is a common neurodevelopmental condition impacting children and adults. Current treatments are limited and there is increasing interest in the use of mindfulness, which is growing in popularity. Previous systematic reviews have typically focused on specific ages, outcome measures or interventions. The current review aimed to take a more comprehensive approach. Title and abstract searches were performed in five databases for randomized and non-randomized controlled trials or pre-post studies with participants with an ADHD diagnosis or a score above a clinically relevant cut-off on a validated ADHD measure. Studies had to measure symptoms of ADHD, global or social functioning. Extraction of key information including participant status (i.e., diagnosis, scale scores, comorbidities, medication use), study design, and outcome measures was conducted. Effect sizes (Hedge’s g) were calculated and where a measure had been used in at least three studies with the same population, meta-analyses were considered. Twenty-nine studies were initially identified, with seven deemed poor quality and removed from further analysis leaving, 22 studies containing data from 1237 children and adults with ADHD along with data from 525 family members. The data indicate possible benefits of mindfulness on self-compassion, quality of life, wellbeing, depression, and anxiety. The findings also suggest that mindfulness may also improve ADHD symptoms, executive function, problematic behaviours, and emotional dysregulation, although results vary by age of patient and measures used. Parent stress and parenting style have received limited attention, highlighting the need for more studies in these areas. Whilst the data presented suggest that mindfulness may be beneficial, the evidence base is not as strong as the popularity of the approach. Many of the studies lacked blind assessment, adequate randomization, or suitable control conditions. As such high-quality controlled studies considering medication, other psychosocial interventions, use of active and in active controls and comorbidity as well as longer follow-up periods, are needed to confirm this.