Background: Current policy in the England suggests that people with intellectual disabilities should, where possible, access mainstream mental health services; this should include access to mainstream therapy services. It is likely that mainstream therapists will need training and support to work with people with intellectual disabilities. Method: Sixty-eight therapists working in an English Improving Access to Psychological Therapies (IAPT) service received one- or 2-day training on working with people with intellectual disabilities. Measures of confidence, general therapeutic self-efficacy and attitudes to people with intellectual disabilities' use of mainstream mental health services were completed pre-training, post-training and at 3-month follow-up; at which time, 12 participants were interviewed about the impact of the training on their practice. Results: There was a significant positive change in all measures immediately post-training which was maintained at 3-month follow-up. Conclusions: Training considerations for mainstream therapists who may work with people with intellectual disabilities are discussed.