Background: Workforce development is a key element for building the capacity to effectively address priority population nutrition issues. On-the-job learning and mentoring have been proposed as strategies for practice improvement in public health nutrition; however, there is limited evidence for their effectiveness. Methods: An evaluation of a mentoring circle workforce development intervention was undertaken. Thirty-two novice public health nutritionists participated in one of three mentoring circles for 2 h, every 6 weeks, over a 7-month period. Pre- and post-intervention qualitative (questionnaire, interview, mentor diary) and quantitative (competence, time working in public health nutrition) data were collected. Results: The novice public health nutritionists explained the intervention facilitated sharing of ideas and strategies and promoted reflective practice. They articulated the important attributes of the mentor in the intervention as having experience in and a passion for public health, facilitating a trusting relationship and providing effective feedback. Participants reported a gain in competency and had an overall mean increase in self-reported competence of 15% (range 3-48% change; P < 0.05) across a broad range of competency elements. Many participants described re-orienting their practice towards population prevention, with quantifiable increases in work time allocated to preventive work post-intervention. Conclusions: Mentoring supported service re-orientation and competency development in public health nutrition. The nature of the group learning environment and the role and qualities of the mentor were important elements contributing to the interventions effects. Mentoring circles offer a potentially effective strategy for workforce development in nutrition and dietetics.