Background: Carers can have a significant impact supporting people with intellectual disabilities to make healthy lifestyle choices. This study examines carers' training needs on diet and physical activity. Methods: A cross-sectional survey was undertaken of the knowledge and perceptions of carers supporting adults with intellectual disabilities. An interviewer administered questionnaire was used to examine carer knowledge of public health recommendations on diet and physical activity; perceptions of the benefits of healthy diets and physical activity levels; and the carer views on the barriers to change experienced by individuals with intellectual disabilities. Results: Sixty-three carers took part in the study. They generally had a low level of knowledge around public health recommendations on diet and physical activity. Greater importance was attributed to the health benefits of diet than physical activity. Carers rated intrapersonal barriers to change within the person with intellectual disabilities as more important, than interpersonal or external barriers to change, with significant differences in perceived barriers relevant to diet and physical activity. Conclusions: Carers supporting adults with intellectual disabilities have significant training needs relevant to promoting healthy lifestyles. This highlights the opportunity to promote health improvement via the development, and provision, of effective training initiatives.