Background: Internet-delivered cognitive behaviour therapy may increase access by young adults to evidence-based treatments for anxiety and depression. Objective: The aim of this study was to compare the efficacy of an Internet-delivered cognitive behaviour therapy intervention designed for adults aged 18–24 years, when delivered in clinician-guided versus self-guided formats. Design: The intervention, the Mood Mechanic Course, is a transdiagnostic treatment that simultaneously targets symptoms of anxiety and depression using cognitive and behavioural skills. The brief intervention comprised four lessons, delivered over 5 weeks. Following a brief telephone interview, young adults ( n = 191) with symptoms of anxiety and depression were randomly allocated to either (1) clinician-guided treatment ( n = 96) or (2) self-guided treatment ( n = 95). Results: At post treatment, large reductions (average improvement; clinician guided vs self-guided) were observed in symptoms of anxiety (44% vs 35%) and depression (40% vs 31%) in both groups. Significant improvements were also observed in general psychological distress (33% vs 29%), satisfaction with life (18% vs 15%) and disability (36% vs 29%). No marked or consistent differences in clinical outcomes emerged between conditions at post-treatment, at 3-month or 12-month follow-up. Satisfaction was high with both treatment formats, but slightly higher for clinician-guided treatment. Conclusion: These results indicate the potential of carefully developed Internet-delivered cognitive behaviour therapy interventions for young adults with anxiety and depression provided in either self or therapist-guided format. Further large-scale research is required to determine the short- and long-term advantages and disadvantages of different models of support.