Watson was born at Higher Broughton, near Salford, Lancashire, the only son of David Watson, a pioneering metallurgist. He was educated at Manchester Grammar School and the University of Manchester. He specialised in geology and began to study plant fossils in coal deposits. In 1907, his final year, he published an important paper on coal balls with Marie Stopes (who had an early career as a paleobotanist); after graduating with first class honours he was appointed as a Beyer fellow at Manchester and went on to complete his MSc in 1909.
After his MSc, Watson continued to develop his wide interest in fossils and studied intensively at the British Museum of Natural History in London, and on extended visits to South Africa, Australia, and the United States. In 1912 he was appointed as a Lecturer in Vertebrate Palaeontology, at University College London by Professor James Peter Hill.
His academic work was eventually interrupted in 1916 by World War I when he took a commission in the Royal Naval Volunteer Reserve. He was later transferred to the nascent Royal Air Force where he worked on balloon and airship fabric design.
After World War I, Watson returned to academic study and in 1921 he succeeded Hill as the Jodrell Professor of Zoology and Comparative Anatomy and the curator of what is now the Grant Museum of Zoology at UCL. He devoted his energy to the development of the Zoology department there and consolidated his position as a respected academician. In 1922 he was elected a Fellow of the Royal Society, where he gave the Croonian Lecture in 1924. Four years later, he was invited to give the Romanes Lecture at the University of Oxford; he spoke on "Paleontology and the Evolution of Man".
He was appointed to the British government's Agricultural Research Council in 1931, which involved spending time in the United States where he lectured at Yale University in 1937. At the outbreak of World War II he returned to Britain to supervise the evacuation of the UCL Zoology department to Bangor, Wales, and then became Secretary of the Scientific Subcommittee of the Food Policy Committee of the War Cabinet.
After the war he continued to teach and to travel widely. He received many awards and academic honours including the Darwin Medal from the Royal Society, the Linnean Medal from the Linnean Society, the Wollaston Medal from the Geological Society of London, and honorary degrees from many universities in Britain and elsewhere. In 1941 Watson was awarded the Mary Clark Thompson Medal from the National Academy of Sciences. He retired from his chair in 1951, but continued to study and publish at UCL until his full retirement in 1965. He was awarded the Linnean Society of London's prestigious Darwin-Wallace Medal in 1958.
His scientific research, besides his early original work on fossil plants and coal balls, was chiefly concerned with vertebrate palaeontology, especially fossil reptiles. He amassed a large collection of fossils from his wide travels to Africa and Spain.
The Science library, known as the DMS Watson library, of University College London is named in his honour. It is UCL's second largest library and is in Malet Place adjacent to the Petrie Museum of Egyptian Archaeology.
|“||the theory of evolution itself, a theory universally accepted not because it can be proved by logically coherent evidence to be true but because the only alternative, special creation, is clearly incredible.||”|
This quotation of Watson is often used in Creationist writings in an attempt to show that Watson, and thus by extension promoters of evolution in general, dismiss creationism due to antitheistic bias. A slightly different version of the quotation, derived from a secondhand source, is sometimes used (e.g., by C. S. Lewis):
|“||[Evolution is] accepted by zoologists not because it has been observed to occur or . . . can be proved by logically coherent evidence to be true, but because the only alternative, special creation, is clearly incredible.||”|
Sometimes the words in square brackets are incorrectly incorporated into the quotation, and/or the ellipsis is omitted.
Watson's original statement first appeared in a 1929 article, "Adaptation," in the journal Nature: The second version of the quotation, given above, is formed by combining parts of two similar passages in Watson's paper, one from the first page and one from the third. The first passage reads:
|“|| Evolution itself is accepted by zoologists not because it has been observed to occur or is supported by logically coherent arguments, but because it does fit all the facts of taxonomy, of paleontology, and of geographical distribution, and because no alternative explanation is credible.||”|
The second passage reads:
|“|| If so, it will present a parallel to the theory of evolution itself, a theory universally accepted not because it can be proved by logically coherent evidence to be true, but because the only alternative, special creation, is clearly incredible||”|
The ellipses in the second version of the standard quotation from Watson elide his statement in  that evolution fits "all the facts" of taxonomy, paleontology, and geographical distribution. They also omit his statement, which directly follows quotation  above, that "Whilst the fact of evolution is accepted by every biologist, the mode in which it has occurred and the mechanism by which it has been brought about are still disputable."
Watson thus considered evolution a fact, belief in which was supported by its fit to a wide range of other facts. He thought "special creation" unbelievable and the mechanisms of evolution disputable (his article was devoted to emphasizing the inadequacy of contemporary theories of adaptation, and mentions "special creation" only in passing). This was in 1929, several years before the inception of evolutionary biology's Modern Synthesis, which integrated Mendelian genetics into Darwinian thought and produced widespread scientific consensus about basic evolutionary mechanisms. Stephen Jay Gould describes 1900–10 as “the period of greatest agnosticism and debate about evolutionary mechanisms” and adds that even the 1920s were still “not happy times of consensus for evolutionary theory in general.” Julian Huxley referred to that era as "the eclipse of Darwinism".
When it was made, over 80 years ago, Watson's complaint that the mechanisms of evolution were poorly understood was accurate. His statement that evolution was believed "because the only alternative, special creation, is clearly incredible" was a provocative exaggeration, contradicted by his own remarks (i.e., evolution already "fit all the facts" of several major knowledge fields).