Trained lay volunteers may have value in supporting lifestyle change programs in the prevention of type 2 diabetes, but the potential health benefits (or harms) experienced by these lay volunteers have not been well described. This is important, as this is an appealing model in terms of workforce planning. The aim of the prespecified quantitative study reported here, was to examine the possible health benefits or harms experienced by these trained lay volunteers with type 2 diabetes. In a large type 2 diabetes prevention program, we recruited and trained 104 lay volunteers with type 2 diabetes themselves, to act as diabetes prevention mentors and codeliver the lifestyle intervention. Mentors made motivational telephone calls to 461 participants randomized to one of the trial arms to encourage lifestyle changes. Weight, diet, physical activity, well-being, quality of life, diabetes-specific self-efficacy, and glycaemic control were measured at baseline, 12 and 24 months. Average mentor age was 62.0 years, 57 (54.8%) were male, 92 (88.5%) were overweight or obese (BMI>30 kg/m[superscript 2]). At 12 months, mentor dietary behaviors (fat and fiber intake) improved significantly, sedentary time spent fell significantly, and diabetes specific self-efficacy scores significantly increased. These significant improvements, with no evidence of harms, suggest lay volunteers with type 2 diabetes codelivering a lifestyle intervention, may themselves experience health benefits from volunteering.