Emerging infections are a potential risk during mass gathering events due to the congregation of large numbers of international travellers. To mitigate this risk for the London 2012 Olympic and Paralympic Games, a sentinel surveillance system was developed to identify clusters of emerging infections presenting as undiagnosed serious infectious illness (USII) in intensive care units (ICUs). Following a six month pilot period, which had begun in January 2011, the surveillance was operational for a further 18 months spanning the Games. The surveillance system and reported USII cases were reviewed and evaluated after this 18 month operational period including assessment of positive predictive value (PPV), timeliness, acceptability and sensitivity of the system. Surveillance records were used to review reported cases and calculate the PPV and median reporting times of USII surveillance. Sensitivity was assessed through comparison with the pilot period. Participating clinicians completed a five-point Likert scale questionnaire about the acceptability of surveillance. Between 11 July 2011 and 10 January 2013, 34 cases were reported. Of these, 22 remained classified as USII at the time of the evaluation, none of which were still hospitalised. No clusters were identified. The 22 USII cases had no association with the Games, suggesting that they represented the background level of USII in the area covered by the surveillance. This corresponded to an annualised rate of 0.39 cases/100,000 population and a PPV of 65%. Clinicians involved in the surveillance reported high acceptability levels. The USII surveillance model could be a useful public health tool in other countries and during mass gathering events for identifying potential clusters of emerging infections.