David Millin attended the South African College High School in Cape Town and passed the matriculation examination of the University of the Cape of Good Hope (in the first class) in 1906, at the age of sixteen. Continuing his studies at the South African College, he was awarded the degree Bachelor of Arts (BA) with honours in chemistry by the university in 1909. During the next two years he was at University College, London, and from about 1915 was associated with the Chamber of Mines in Johannesburg. In the nineteen-twenties he was appointed as lecturer in the Department of Metallurgy and Assaying, University of the Witwatersrand, under Professor G.H. Stanley*. He remained with the department until his retirement in 1955.
Millin became a member of the South African Chemical Institute in 1915, but was no longer a member by 1934. He published a paper on 'The assay of chromite' in the South African Journal of Science (1934, Vol. 31, pp. 177-183).
Millin married Mary Hoda, born Judes, in Johannesburg. They had two sons.