Moseley was born in Weymouth, England. In 1906 he entered Trinity College of the University of Oxford, and on graduation from that institution went to Manchester University to work with Ernest Rutherford. For his first year at Manchester, he had a full teaching load, but after a year he was relieved of his teaching duties and began full-time research.
In 1913, by using x-ray spectra obtained by diffraction in crystals, he found a systematic relation between wavelength and atomic number. Previous to this, atomic numbers had been thought of as an arbitrary number, based on sequence of atomic weights, but altered (e. g., by Dmitry Ivanovich Mendeleev) when necessary to put an element in the appropriate place in the periodic table. Moseley's discovery showed that atomic numbers were not arbitrary but had an experimentally measurable basis. In addition, Moseley showed that there were gaps in the sequence at numbers 43 and 61 (now known to be radioactive, non-naturally-occurring, technetium and promethium, respectively). Thus Moseley, in the tradition of Mendeleev, predicted two elements.
In 1914 he resigned at Manchester to return to Oxford to pursue his research, but when World War I broke out, he enlisted in the Royal Engineers. He fought at Gallipoli, where he was killed in action.
Relatively young at death, Moseley could in many scientists' opinions have contributed much to the knowledge of atomic structure had he lived a bit longer.