Background: Individual service funds (ISFs) in England aim to provide self-directed support without the challenge of procuring support and managing a budget. However, few local authorities offer ISFs and some do not offer more choice and control than a council-managed budget. Methods: This participatory realist review followed RAMESES publication standards. We developed and refined theories using published and grey literature, expert stakeholders and personal narratives (written and video case studies). Results: We identified eight initial programme theories, forming a programme theory explaining how ISFs generate successful outcomes for adults with intellectual disabilities. These included: involvement in support planning; accessible budget information; flexible use of budgets; outcome-focused support planning; 'live' support plans; non-traditional support; positive risk management and trusting relationships. Conclusion: Our programme theory elucidates causal pathways for successful ISF outcomes, connecting mechanisms to contextual factors. This guides ISF development and implementation and helps adults with intellectual disabilities make informed decisions about ISFs.