Improvements in parenting skills do not necessarily lead to improvements in child and parent outcomes for children with attention deficit and hyperactivity disorder (ADHD). Cognitive factors such as parental cognitions and parenting self-efficacy may influence such outcomes. Clinical research on the effects of cognitive-behaviour therapy (CBT) on parental cognitions, parenting self-efficacy and mental health in Chinese parents of children with ADHD is lacking. This quasi-experimental study intended to assess the effects of a culturally attuned group CBT treatment on the mental health and quality of life of Chinese parents aged 26–58 years, with children with ADHD in Hong Kong. Participants in the experimental group received group CBT and standard treatment and their counterparts in the control group received standard treatment. Primary outcome variables of mental health and quality of life and process variables including parenting stress, parenting efficacy and dysfunctional attitudes were measured at baseline, post-treatment and 3-month follow-up. Data were analysed using 2 × 3 ANCOVAs, Cohen’s d statistics and multiple mediation analyses. Group by time interactions were found for most outcome variables. Effect size statistics indicated that group CBT had small to moderate effects on most outcome variables at post-treatment and follow-up assessments. The effect of CBT on mental health was mediated by the reduction in parenting stress and the effect of CBT on quality of life was mediated by the reductions in parenting stress and dysfunctional attitudes. Given its potential benefit, it is worth considering incorporating this group CBT programme into interventions for Chinese parents with ADHD children.