Born in Worcester, Massachusetts, Bemis received his B.A. degree in 1912 from Clark University. Influenced by George Hubbard Blakeslee of the Clark faculty, Bemis also acquired an A.M. from Clark the following year. In 1916 he was granted his Ph.D. by Harvard University. He first taught at Colorado College from 1917 to 1921. From 1921-1923, he taught at Whitman College in Walla Walla, Washington. In 1923-1924, he served as a research associate at the Carnegie Institution of Washington's Division of Historical Research. Bemis joined the faculty at George Washington University in 1924, remaining there a decade, and accepted the history department's chairmanship in 1925. From 1927 to 1929, he led the Library of Congress's European Mission. He left George Washington University in 1934, first serving as lecturer at Harvard University for the 1934-1935 academic year while James Phinney Baxter III was on research leave. Then, in 1935, he took up his position at Yale University, where he remained through the end of his career. He was first the Farnham Professor of Diplomatic History and then in 1945 became the Sterling Professor of Diplomatic History and Inter-American Relations. In 1958, he was elected a Fellow of the American Academy of Arts and Sciences. He retired in 1960, and served as president of the American Historical Association in 1961. His presidential address for the AHA engaged the topic of "American Foreign Policy and the Blessings of Liberty." He died in Bridgeport, Connecticut aged 81.
Mark Gilderhus says Bemis was a "founding father" of the field of diplomatic history in the United States. His tone was nationalistic, typically blaming America's antagonists for conflicts, but he rose above jingoism and provided analysis which ran counter to State Department views. For Bemis, the great achievement US-Latin American relations was Franklin Roosevelt's Good Neighbor policy. He praised it for unifying the Pan American nations, along with the U.S. leadership against Fascists and Nazis. During the Cold War, Bemis saw Latin America as a minor backwater of diplomacy.
Bemis was a strong writer, and his works attracted prizes for their quality. He also impressed upon his students the importance of good writing, a trend which they frequently passed down to their own students. He won the Pulitzer Prize twice. Bemis's books include Jay's Treaty: A Study in Commerce and Diplomacy (1924 and later reprint editions), which won the Knights of Columbus Historical Prize. His Pinckney's Treaty: America's Advantage from Europe's Distress, 1783-1800 (1926) was the published version of the Albert Shaw Lectures on Diplomatic History, and was the winner of the 1927 Pulitzer Prize for History. His other works include The Latin American Policy of the United States (1943) and The Diplomacy of the American Revolution (1935).
His 18-volume series The American Secretaries of State and Their Diplomacy appeared first in ten volumes (published by Knopf in 1927-1929) covering Robert R. Livingston to Charles Evans Hughes. These were reprinted in 1958, and the success of the series prompted the creation of a further eight volumes, covering Frank B. Kellogg to Christian Herter, published through 1972. He also authored a well-known textbook on diplomatic history that first appeared in 1936 and went through four revisions.