In 1921, Merrell became a diplomat for the United States. From March 1924 to October 26, he served as the Chargé d’Affaires ad interim to
Haiti. In April 1935, Merrell became the First Secretary of the United States Legation in
Peiping,
China. On September 20, he was promoted to consul general and October 25, he was appointed to simultaneously serve as the Consul General in
Kabul,
Afghanistan. In 1945, Merrell was appointed by President
Franklin D. Roosevelt to serve as the United States
Minister to India and was approved by the
United States Senate on February 27. On April 15, 1947, President
Harry S. Truman appointed Merrell to replace
Felix Cole as the
Envoy Extraordinary and Minister Plenipotentiary.
Henry F. Grady was appointed to succeed Merrell as the United States Ambassador to India. On May 14, the United States Senate approved his nomination and he presented his credentials on January 1, 1948. On June 28, 1949, he was promoted to Ambassador Extraordinary and Plenipotentiary and served until March 17, 1951. On April 19, 1951, he was appointed to serve as the
United States Ambassador to Afghanistan. He presented his credentials on June 28, 1951, and served until May 3, 1952. ==Later life==