Clive Day was born in Hartford on February 11, 1871, and received his B. A. from Yale in 1892; he came from a long line of famous Yale men — indeed the direct line of Day's ancestry had had every generation represented at Yale since the founding of the College. Following a year of graduate studies at the University of Berlin, Day went to California and taught history and economics at Berkeley. Then, after another year of postgraduate work at the universities of Paris and Berlin, he returned to Yale and completed his Ph. D. requirements in 1899. That same year he embarked on his long and distinguished academic career at this university: at the time of his retirement in 1936 Day was Knox Professor of Political Economy and was the author of Policy and Administration of the Dutch in Java (1904); A History of Commerce (1907); The Question of the Balkans (1920); and A History of Commerce in the United States (1925).He wrote several books, served on many university committees, and was a member of the Connecticut Unemployment Commission, 1932-1933.

There was only one significant break in Day's academic career: late in 1917 he was asked to join Colonel E. M. House's "Inquiry" group as an adviser on economic questions relating to Austria-Hungary. During 1918 Day produced a large number of reports on Eastern and Central Europe which were meant to assist the intended American negotiators at the Paris Peace Conference. (Copies of many of these reports can be found in the Yale University Library's "The Inquiry Papers" [Mss. Group 8].) By December 1918, when the nucleus of the "Inquiry" experts had rejoined Colonel House in Paris, Day had emerged - faute de mieux, as he was always the first to point out - as the Chief of the entire Balkan Section of the Division of Territorial, Economic, and Political Intelligence of the American Commission to Negotiate Peace. Day remained in France until the signing of the treaty of Versailles and was directly involved in some of the most complicated debates and discussions bearing upon the territorial settlements imposed by the Conference on various segments of the former Ottoman and Austro-Hungarian empires. Indeed, Day's letters from Paris contain a mine of information about the peace conference in general and about the inner workings of the American Delegation more particularly. After his return from Europe Day contributed a chapter on "The Atmosphere and Organization of the Peace Conference" in the collection of essays edited by E. M. House and Charles Seymour entitled What Really Happened at Paris (1921). He died in 1951.