William Henry Mahoney Christie (1845-1922) was born at Woolwich on 1 October 1845, son of Samuel Hunter Christie, Professor of Mathematics at the Royal Military Academy, Woolwich. He attended King's College School, London, and matriculated at Trinity College, Cambridge, in 1864, where he studied mathematics (B.A., 1868), and was elected a Fellow in 1869.

In 1870 Christie was selected by George Airy, the Astronomer Royal, as the new Chief Assistant at Greenwich. After taking up his position, he instituted the daily photographic observations of the Sun and spectroscopic observations of the Sun and stars. He invented a mechanical paper drum in 1873 for recording the observation of star declinations, and designed a new spectroscope, which came into use in 1877. He also introduced the Observatory magazine, which he continued to edit until 1882, and published several papers on spectroscopy.

In 1881 Christie was appointed Astronomer Royal in succession to Airy, and began work on expanding and updating the work of the Observatory. Solar photography at Greenwich was increased and improved, and other observatories were asked to provide plates for days when the weather conditions at Greenwich were unfavourable to photography. Christie made improvements to the Observatory's instrumentation, procuring a 28-inch refractor and 13-inch photographic refractor and instigating the purchasing or refurbishing of other telescopes. He also oversaw changes to the Greenwich site, with the construction of new buildings and the expansion of existing ones. Some of the most significant reforms of this period related to staffing. The number of staff was greatly increased, and changes were made to improve the methods of recruitment. In 1896 Christie persuaded the Admiralty to allow him two Chief Assistants, leading to the appointment of P.H. Cowell to support the existing assistant, F.W. Dyson. Other changes included improvements to work rotas and the employment of women as computers for the first time. Away from Greenwich, Christie contributed to many international conferences and engaged in the discussions that led to the adoption of Greenwich as the international Zero Meridian. He also joined expeditions to observe solar eclipses, including trips to Japan (1896), India (1898), Portugal (1900) and Tunisia (1905).

Christie was married in 1881 to Violette Mary Hickman (d. 1888), by whom he had two sons, one of whom died at a young age. His elder son, Harold, became a lawyer and lived with his father at Greenwich until his marriage in 1921. Christie was an active member of the Royal Astronomical Society, being elected Fellow in 1871, and serving as Secretary, 1880-1882, and President, 1888-1890. He was also a Fellow of the Royal Society, serving on its Council three times, including twice as Vice-President. In 1902 he was awarded an Honorary D.Sc. by Oxford University, and in 1904 he received his knighthood. He retired from the Royal Observatory in 1910, and spent his later years travelling extensively. He died at sea on 22 January 1922, while travelling to Morocco.