Simulation exercises are an important part of emergency preparedness activities for the healthcare community but evidence of their impact on the response to real major incidents is limited. This project studied the impact of health emergency preparedness exercises (HEPEs) on the response to a mass casualty terrorist incident. The mixed methods study design was adopted comprising an on-line survey and follow up individual interviews. Participants were healthcare staff who took part in responses to three major terrorist incidents in the UK in 2017. Descriptive statistics and analysis of variance were undertaken with quantitative data. Content and thematic analysis methods were used for qualitative data analysis. The online survey generated 86 responses; 79 (92%) were from the responders to the Manchester Arena bombing. Twenty-one survey respondents shared their experiences in in-depth interviews. Healthcare staff who took part in HEPEs felt better prepared to respond than those who did not attend an exercise. The most commonly reported benefits from HEPEs were awareness of major incident plans and having the opportunity to practice responding to a similar scenario in the recent exercise. Specific benefits included: improved coordination of the response through adherence to recently practiced incident plans confidence with response roles; real-time modifications of the response and support provided to staff who did not take part in exercises. Exercise recency was highlighted as an important facilitating factor. The study provides strong objective evidence that the response to a mass casualty terrorist incident was enhanced by training and service development achieved through HEPEs.