Born David Abner Moscovitz in New York on May 31, 1907, Morse graduated from Rutgers University in 1929, where he was a member of the Cap and Skull Society,[2] and from the Harvard Law School in 1932.

In 1937 Moscovitz changed his surname to Morse, "motivated by instances of anti-Semitic resentment he had experience[d] in both his private and professional life."

From June 1943 to April 1944, Morse served as Captain in North Africa, Sicily and Italy, where he was appointed Chief of the Labor Division of the Allied Military Government (1945).

He drafted and put into effect the labour policy and programme in Sicily and Italy for the British and United States Governments and armies.

At the 105th session of the Governing Body in San Francisco in June 1948, he was unanimously elected director-general for a ten-year term.