Transdiagnostic internet-delivered cognitive behavioural therapies (iCBT) are effective for treating anxiety and depression, but there is room for improvement. In this study we developed a new Mindfulness-Enhanced iCBT intervention by incorporating formal and informal mindfulness exercises within an existing transdiagnostic iCBT program for mixed depression and anxiety. We examined the acceptability, feasibility, and outcomes of this new program in a sample of 22 adults with anxiety disorders and/or major depression. Participants took part in the 7-lesson clinician-guided online intervention over 14 weeks, and completed measures of distress (K-10), anxiety (GAD-7), depression (PHQ-9), mindfulness (FFMQ) and well-being (WEMBWS) at pre-, mid-, post-treatment, and three months post-treatment. Treatment engagement, satisfaction, and side-effects were assessed. We found large, significant reductions in distress (Hedges g = 1.55), anxiety (g = 1.39), and depression (g = 1.96), and improvements in trait mindfulness (g = 0.98) and well-being (g = 1.26) between baseline and post-treatment, all of which were maintained at follow-up. Treatment satisfaction was high for treatment-completers, with minimal side-effects reported, although adherence was lower than expected (59.1% completed). These findings show that it is feasible to integrate online mindfulness training with iCBT for the treatment of anxiety and depression, but further research is needed to improve adherence. A randomised controlled trial is needed to explore the efficacy of this program.