After a fact-finding visit to the United States in 1942, Perrin realised that its government had, at last, recognised the significance of the atomic bomb, and saw the potential of that country now that it had properly mobilised.
He was also charged with understanding the status of the German atomic bomb programme through the military intelligence services and their spy network, including interviewing the physicist Niels Bohr after he fled occupied Denmark.
He was also tasked with documenting the story of Britain's role in developing the atomic bomb to counterbalance the published American account, which was thought at the time to have not properly acknowledged the British work.
After the war, Lord Portal was appointed to the new government post of Controller of Production (Atomic Energy) at the Ministry of Supply with Perrin as his hands-on deputy.
In this position he worked hard to secure the finances of the organisation, and directed it in the expansion of university research and training across medicine, pharmacology and allied disciplines.