Abstract Background delirium is a frequent complication of hospital admission for older people and can be reduced by multicomponent interventions, but implementation and delivery of such interventions is challenging. Objective to investigate fidelity to the prevention of delirium system of care within a multicentre, pragmatic, cluster randomised, controlled feasibility trial. Setting five care of older people and three orthopaedic trauma wards in eight hospitals in England and Wales. Data collection research nurse observations of ward practice; case note reviews and examination of documentation. Assessment 10 health care professionals with experience in older people’s care assessed the fidelity to 21 essential implementation components within four domains: intervention installation (five items; maximum score = 5); intervention delivery (12 items; maximum score = 48); intervention coverage (three items; maximum score = 16); and duration of delivery (one item; maximum score = 1). Results the mean score (range) for each domain was: installation 4.5 (3.5–5); delivery 32.6 (range 27.3–38.3); coverage 7.9 (range 4.2–10.1); and duration 0.38 (0–1). Of the 10 delirium risk factors, infection, nutrition, hypoxia and pain were the most and cognitive impairment, sensory impairment and multiple medications the least consistently addressed. Overall fidelity to the intervention was assessed as high (≥80%) in two wards, medium (51–79%) in five wards and low (≤50%) in one ward. Conclusion the trial was designed as a pragmatic evaluation, and the findings of medium intervention fidelity are likely to be generalisable to delirium prevention in routine care and provide an important context to interpret the trial outcomes.