Sir James Dewar was born at Kincardine-on-Forth, Scotland, on 20 September 1842, the youngest son of Thomas Dewar, vintner, and his wife, Ann Eadie, daughter of a shipowner. At the age of ten his schooling was interrupted by an attack of rheumatic fever. On recovering he was sent to Dollar Academy, and then in about 1858 to Edinburgh University. In 1869 he was appointed lecturer on chemistry in the Royal (Dick) Veterinary College, where he later became professor. He married Helen Rose Banks in 1871. In 1875 he left Edinburgh for Cambridge, having been elected to the Jacksonian chair of natural experimental philosophy. Two years later he was elected Fullerian professor of chemistry at the Royal Institution of Great Britain, London. He held both these chairs until his death.

Dewar’s contributions to science lay in the discovery of new facts as opposed to the elaboration of theory. During his time at Edinburgh he worked on both organic and inorganic chemistry. Of some interest is his work on the specific heat of Graham's hydrogenium in 1872. During his research and in order to prevent the influx of heat into his calorimeter, Dewar used, the vacuum jacket which later proved indispensable for the liquefaction of gases, and ultimately became the silver coated ‘Thermos’ flask. Whilst at Cambridge Dewar joined George Downing Living in a series of spectroscopic investigations. Some of the work was done at Cambridge, but the Royal Institution, with its superior laboratory facilities, became the centre of Dewar's experimental activities. At the Royal Institution, Dewar implemented the extensive research programme into cryogenics for which he is best known.

The papers recording Dewar and Living’s spectroscopic researches were published in collected form in 1915, and the rest of his papers in 1927. Dewar also engaged in a number of other scientific projects, including experiments resulting in the invention of cordite and work on sewage disposal and water supply. Dewar was elected a Fellow of the Royal Society in 1877 and knighted in 1904. He received many honours from scientific societies in Europe and America.

Dewar died at his home in the Royal Institution, 21 Albemarle Street, on 27 March 1923.