The problem gambling (PG) intervention literature is characterised by a variety of psychological treatments and approaches, with varying levels of evidence (PGRTC in Guideline for screening, assessment and treatment in problem and pathological gambling. Monash University, Melbourne, 2011). A recent PG systematic review (Maynard et al. in Res Soc Work Pract, 2015. doi:10.1177/1049731515606977) and the success of mindfulness-based interventions to effectively treat disorders commonly comorbid with PG suggested mindfulness-based interventions may be effective for treating PG. The current study tested the effectiveness of three interventions to treat PGs: 1. case formulation driven Cognitive Behaviour Therapy (CBT); 2. manualised CBT; and 3. mindfulness-based treatment. All three interventions tested returned large effect size improvements in PG behaviour after seven sessions (Cohen’s d range 1.46–2.01), at post-treatment and at 3 and 6-month follow-up. All of the interventions were rated as acceptable by participants at post-treatment. This study suggests that the mindfulness-based and TAU interventions used in the current study appear to be effective at reducing PG behavior and associated distress and they also appear to generalise to improvements in other measures such as quality of life-mental functioning and certain mindfulness facets more effectively than the manualised form of CBT utilised used here. Secondly, a brief mindfulness intervention delivered after psycho-education and a brief CBT intervention may be a useful supplement to traditional CBT treatments by addressing transdiagnostic processes such as rumination and thought suppression. Thirdly, CBT interventions continue to report effectiveness in reducing PG behaviour and associated distress consistent with the prevailing literature and clinical direction.