Alexander (‘Alec’) Walter Merrison was born in Wood Green, London, on 20 March 1924. He was the only child of Henry Walter Merrison, a fitter's mate, who rose to be a service manager in the local Gas Board and a respected Chairman of the Tottenham Group of Hospitals, and of Violet Henrietta Merrison (née Mortimer) the daughter of an Ipswich family. Alec attended Tottenham Grammar School, then the Grammar School, Enfield, where he took the Higher School Certificate in physics, chemistry and mathematics. He became Captain of the school and is remembered as a fine scholar with a pleasant manner. His qualities of leadership were already evident at a very young age. He was also a choirboy at All Hallows Church, Wood Green, where his lifelong love of music was first developed.
;In 1944 he graduated in physics at King's College, London, when he was just 20 years old, researching radio wave propagation, after which he was ‘placed’ on wartime radar at the Signals Research Development Establishment at Christchurch, the only Englishman and civilian in a group of 26 engineers of the Polish Army in exile. Two years later he requested a transfer to the Atomic Energy Research Establishment at Harwell to participate in research of a more interesting and challenging nature. There he came under the tutelage of O.R. Frisch (F.R.S. 1948) and J.D. (later Sir John) Cockcroft, F.R.S., who were the leading research scientists in nuclear physics. At that time Harwell was the breeding ground for a generation of British physicists; Alec clearly relished this new environment, helping to equip an electron accelerator to produce short pulses of neutrons. His first published papers described how the new technique could be used to study the interaction of neutrons with matter. This was his first experience of the use of particle accelerators as powerful probes to investigate nuclear matter. The technique of neutron scattering from bulk matter is now an important discipline in its own right, and the genesis of the current world-leading facility (ISIS) at the Rutherford Appleton Laboratory can be traced back to these pioneering experiments in which Alec played a major role.