To explore the value of a peer mentoring program for first year medical interns and to assess the demand for and benefits of such a program in an Australian hospital. Randomised controlled study of the impact on first year interns of peer-led mentoring by second and third year interns, undertaken during 2015 at the Royal Perth Hospital, a tertiary teaching hospital. Methods and main outcome measure: Interns were recruited and randomised 1:1 to being assigned or not assigned a mentor. Qualitative outcome data were collected in semi-structured interviews and focus groups at 12 months to assess psychosocial wellbeing and job satisfaction. Fifty-three of 79 interns (67%) applied to participate in the program. Twenty-six mentor-mentee pairs matched by sex and career preferences were established; 27 interns were allocated to the control group. Iterative data analysis identified two major themes related to the value of the mentorship program: aiding navigation through the complex health care system, and enhancing a sense of community. Participants with mentors reported high satisfaction with the program and a positive impact on stress levels, morale, sense of support, job satisfaction, and psychosocial wellbeing compared with participants without mentors. An optional peer mentoring program enhances junior doctor support structures, builds a sense of community, and helps participating interns navigate their new professional environment. Our trial provides a feasibility model that could be adapted to local conditions, regionally or nationally. Australian New Zealand Clinical Trials Registry, ACTRN12618000455268; 29 March 2018 (retrospective).