There is some evidence that cognitive flexibility negatively impacts cognitive restructuring skill acquisition with brief training; however, there is little understanding of how this relates to learning cognitive restructuring over the course of a therapy program, and how it relates to overall treatment outcome. This study assessed the impact of cognitive flexibility on cognitive restructuring skill acquisition following group CBT, and on treatment outcome, along with changes in cognitive flexibility over treatment. 44 older participants with anxiety and depression completed self-report and neuropsychological tests of cognitive flexibility and a clinical interview at pre and post-treatment. Qualitative and quantitative measures of cognitive restructuring were completed at post-treatment. Pre-treatment cognitive flexibility was not related to the quality of cognitive restructuring at post-treatment or overall treatment outcome. However, it did predict reduction in subjective units of distress from using cognitive restructuring and therapist ratings of cognitive restructuring ability at post-treatment. Few participants showed changes in cognitive flexibility over treatment. Those with poorer cognitive flexibility may not find cognitive restructuring as useful to alleviate emotional distress as those with better cognitive flexibility. However, those with poorer cognitive flexibility can still benefit from standardised CBT, even if their use of cognitive restructuring is less effective.