D. Campbell Watt qualified as Bachelor of Medicine (MB) and Master in Surgery (CM) at the University of Glasgow in 1887. After practising at Ferryhill, Durham, for some time he came to Natal in 1889 and settled in Pietermaritzburg. He remained there for his entire career and became a prominent local member of his profession. On 7 October 1894 he was appointed district surgeon and district health officer of the Umgeni Division, with his headquarters in Pietermaritzburg. He still held these positions in 1910. Also in 1894 he was elected as the first honorary secretary and treasurer of the newly established Pietermaritzburg Medical Society. He served the society in these positions until May 1897, when it resolved itself into the Pietermaritzburg Branch of the (second) South African Medical Association, which had been founded earlier that year. Watt then served as the honorary secretary and treasurer of this association (not just the Pietermaritzburg branch). In 1896 he became a member of the first Natal Medical Council and served as its secretary from 1896 until at least 1909. He continued serving as a member of the council for many years and was its president by 1926
Watt sent a package of medicinal plants from Natal to Professor Cash at the University of Aberdeen in 1899. He was awarded the degree Doctor of Medicine (MD) in 1904. In 1908 he published a long letter entitled "Death by lightning" in the Transvaal Medical Journal (Vol. 4, pp. 129-130). It dealt with the effects of lightning strikes on the human body. Later he wrote an obituary of Dr James Hyslop* which was published in the Report of the South African Association for the Advancement of Science for 1917 (pp. 312-314).
In 1906 Watt joined the South African Association for the Advancement of Science and was still a member in 1918. In 1906 he was also president of the Caledonian Society of Pietermaritzburg. In 1924 he retired from the military establishment with the rank of lieutenant-colonel. He married Johanna Catherine Anderson in Pietermaritzburg on 14 February 1893 and they had two sons. The diaries he kept during the Anglo-Boer War (1899-1902) and the South West Africa campaign during World War I (1914-1918) were presented to the Cory Library, Grahamstown by his grandson, Mr Andrew Campbell-Watt.