Aim: To achieve a deeper understanding of the results of a primary randomised controlled trial to clarify the potential effective mechanisms and barriers of a peer-mentor intervention.

Design: Mixed methods process-outcome evaluation of the intervention.

Method: Qualitative and quantitative data were collected during the intervention in a during-trial set-up, that is, a convergent design.

Results: The qualitative and quantitative findings mostly confirmed and expanded each other, identifying several mechanisms that facilitate the effectiveness of peer-mentor support during cardiac rehabilitation, such as mentors' experience-based knowledge and motivation. However, barriers related to lifestyle changes among older, vulnerable patients (e.g., mentee concerns about heart-healthy diets) and psychological outcomes (e.g., mentees' resilience) may minimise the effectiveness.

Conclusion: Peer-mentoring holds potential for supporting older, vulnerable patients during cardiac rehabilitation. However, ensuring that peer-mentors are well-suited for their role and capable of providing motivational, experience-based support is crucial, as is the need for tailored mentorship and consideration of specific patient populations needing mentor-supported cardiac rehabilitation.

Implications and impact: Cardiac rehabilitation faces challenges due to high drop-out rates, particularly among older individuals, females, and vulnerable patients. Peer mentoring, a low-cost intervention, holds promise for supporting these groups in cardiac rehabilitation programmes.

Reporting method: The study adheres to the 'Systematic Development of Standards for Mixed Methods Reporting in Rehabilitation Health Sciences Research', 'Good Reporting of A Mixed Methods Study' and 'Template for Intervention Description and Replication'.

Patient and public contribution: A group of patients with cardiovascular disease actively contributed to developing and implementing the intervention.

Trial and protocol registration: ClinicalTrials.gov Identifier: NCT04945486-prospectively registered before the first participant was recruited.

Keywords: coronary heart disease; mentors; mixed methods design; nursing; older people; rehabilitation.

© 2025 The Author(s). Journal of Advanced Nursing published by John Wiley & Sons Ltd.