Personalised Care Planning for Older People with Frailty (PROSPER) is a complex intervention comprising four work packages with multiple layers of public involvement. We explored the perceived value of public involvement within PROSPER and evaluated the current structure using the six core standards for public involvement developed by the UK National Institute for Health and Care Research (NIHR). Two lay members trained as peer researchers. They conducted semi-structured interviews with other lay members, the trial and programme managers and chief investigator. Interviews were not audio-recorded. Interview notes were written up and returned to participants for verification. Documentary analysis of minutes was also undertaken. Twelve interviews were conducted. Lay members felt involvement in the programme made them reflect on their own experience of older age and encouraged them to be more involved in future research. Lay member input resulted in tangible benefits, including securing funding. The use of the NIHR framework highlighted areas of strength and weakness in the public involvement structure and underpinned an action plan for improvements in the final work package. Public involvement in PROSPER had both perceived and tangible benefits for individuals, the programme, the NHS and wider society. The NIHR standards framework is a useful tool for evaluating public involvement structures in programmes of complex interventions.