Background: This research examines whether the effectiveness of Dialectical Behavioural Therapy (DBT) differs between online and face-to-face therapeutic modalities, and explores the experience of participation in these modalities. Methods: In Study 1, we compared DBT’s effectiveness between modalities by analysing pre-collected client self-report measures from a London-based charity (N = 59) across two non-randomised groups; online clients and face-to-face clients seen between 2018 and 2021. In Study 2, we conducted semi-structured interviews with a subsample of Study 1 participants (N = 10) with the primary aim of capturing the client experience of each modality, and the secondary aim of capturing the general client experience of DBT. Results: It was hypothesised that emotion dysregulation would reduce following DBT regardless of modality, which results supported. It was also hypothesised that DBT would be less effective online, but no difference was identified. A model of the general client experience was proposed, consisting of five themes: ‘the process of learning’, ‘from feeling alone to a sense of community’, ‘physical factors’, ‘improvements in my life’, and ‘DBT beyond the course’. Differences in experience between modalities were identified, and each contributed to the effectiveness of the therapy in distinct ways. Conclusions: Emotion dysregulation reduces following face-to-face and online DBT. The implications are that practitioners may wish to continue to deliver online DBT beyond the Covid-19 pandemic, thereby reducing costs and increasing accessibility. Trial Registry: The trial has been retrospectively registered with the Open Science Framework (www.osf.io).