He secured a post-graduate position at the Victoria University of Manchester, working under Ernest Rutherford and Niels Bohr on Rutherford's atomic theory. In 1912, his interests developed into using his mathematical skills assisting Henry Moseley on X-ray diffraction. His two 1914 papers on diffraction of X-rays from perfect crystals became often cited classics. In a further paper of 1922, he introduced the mosaic crystal model. On the outbreak of World War I, he was commissioned and sent to France as a censor. A year later William Lawrence Bragg had him transferred to the Royal Engineers to participate in the work on the localisation of enemy artillery by sound ranging. When that research was on a solid footing, he was transferred to the RAF to study aircraft noise. From 1919 to 1922 he was a lecturer and fellow of Christ's College, Cambridge where he worked with R.H. Fowler on statistical mechanics and, what came to be known as, the Darwin–Fowler method. He then worked for a year at the California Institute of Technology before becoming Tait Professor of Natural Philosophy at the University of Edinburgh in 1924, working on quantum optics and magneto-optic effects. He was the first in 1928, to calculate the fine structure of the hydrogen atom under P.A.M. Dirac's relativistictheory of the electron. He was assisted at the university by Dr Robert Schlapp. In 1936 Darwin resigned his post in Edinburgh to become Master of Christ's College, beginning his career as an active and able administrator, becoming director of the National Physical Laboratory on the approach of war in 1938. He served in the role into the post-war period, unafraid to seek improved laboratory performance through re-organisation, but spending much of the war years working on the Manhattan Project co-ordinating the American, British, and Canadian efforts. He was appointed KBE in 1942.
In his spare time, Darwin also served as a wartime vice-president of the Simplified Spelling Society. On his retirement, his attention turned to issues of population, genetics and eugenics. His conclusions were pessimistic and entailed a resigned belief in an inevitable Malthusian catastrophe, as described in his 1952 book The Next Million Years. He first argued in this book that voluntary birth control establishes a selective system that ensures its own failure. The cause is that people with the strongest instinct for wanting children will have the largest families and they will hand on the instinct to their children, while those with weaker instincts will have smaller families and will hand on that instinct to their children. In the long run society will consist mainly of people with the strongest instinct to reproduce. This would ultimately have dysgenic effects. In later years he travelled widely, an enthusiastic collaborator across national borders and an able communicator of scientific ideas. He died at Newnham Grange in Cambridge on New Year's Eve 1962/3; he was cremated at Cambridge Crematorium on 4 January 1963. He and his late wife are commemorated with a memorial at St Botolph's Church, Cambridge; she was cremated, the funeral was in Wimbledon, where she had been living.
