Ramsay came from a scientific background in Glasgow, his father being an engineer and one of his uncles a professor of geology. He studied at Glasgow University (1866–69) and returned there as an assistant in 1872 after postgraduate work in chemistry at Tübingen, where he studied under Robert Bunsen. He was appointed professor of chemistry at University College, Bristol, in 1880 and moved to a similar post at University College, London (1887–1912).

Ramsay's early research was mainly in the field of organic chemistry but in 1892 he came across some work of Lord Rayleigh that dramatically changed the direction of his work. Rayleigh reported that nitrogen obtained from the atmosphere appeared to be denser than nitrogen derived from chemical compounds. Rayleigh's original view of this was that the synthetic nitrogen was probably contaminated with a lighter gas. Ramsay, however, predicted that the atmosphere contained some unknown denser gas. He favored this view for he remembered some experiments performed by Henry Cavendish in 1785 in which he showed that present in the air, after removal of all its oxygen and nitrogen, there remained an unabsorbable 1/20th part of the original. Ramsay experimented with methods of totally removing the oxygen and nitrogen from samples of air and found (1894) that a bubble of gas remained. The gas was identified as a new element spectroscopically by Sir William Crookes. Ramsay and Rayleigh announced the discovery of the element in 1898, naming it argon from the Greek ‘inert’.

In the following year Ramsay heard that in America a strange gas had been discovered by heating uranium ores. Ramsay obtained some of the gas from the mineral cleveite and Crookes was able to establish spectroscopically that this was in fact helium, the element whose spectrum had first been observed in a solar eclipse by Pierre Janssen in 1868.

From the positions of argon and helium in the periodic table of elements it appeared that three more gases should exist. In 1898 Ramsay began the search for these, assisted by Morris Travers. They liquefied argon and by its fractional distillation were able to collect three new gases, which they named neon, krypton, and xenon, from the Greek words for ‘the new,’ ‘the hidden,’ and ‘the strange’. Ramsay completed his work on the inert gases when, in 1904, with R. Whytlaw-Gray he discovered niton (now known as radon), the radioactive member of the series, first isolated by the German physicist Friedrich Dorn (1848–1916) in 1900.

In 1903, with Frederick Soddy, Ramsay demonstrated that helium is continually produced during the radioactive decay of radium. The significance of this was not apparent for some time but its explanation by Ernest Rutherford was to lead to the foundation of the new discipline of nuclear physics.

Ramsay was knighted in 1902 and in 1904 was awarded the Nobel Prize for chemistry. His published works included The Gases of the Atmosphere (1896) and his two-volume Modern Chemistry (1900).