Background: In recent years, cognitive behavioural group therapies (CBGT) have been increasingly deployed as a strategy to increase the efficiency and cost-effectiveness in treatment of common mental health problems. The vast majority of these therapies are disorder specific, but in the last few years there has been growing interest in transdiagnostic CBGT. Aims: The aim of this study was twofold: to evaluate the treatment effects of transdiagnostic CBGT on disorder specific symptoms and what (if any) differences would be observed in the treatment effects with regard to general as opposed to disorder specific symptoms measured pre- and post-treatment. Method: The participants were 233 adult patients diagnosed with depression and/or anxiety disorders. They underwent a 6-week transdiagnostic CBGT. To compare treatment effects on general and disorder specific symptoms, raw scores on all measures were converted to standardized scores. Results: Pre–post differences were significant and there was no evidence that treatment was differentially effective for general and disorder specific symptoms. Effect sizes ranged from medium to large. Conclusion: The 6-week transdiagnostic CBGT is feasible for a wide range of mood and anxiety disorders. The results indicate that low-intensity transdiagnostic group therapies may have similar effects on both general and disorder specific symptoms.