He resigned in 1946 to become Special Assistant to the Assistant Secretary-General for Economic Affairs Department of the United Nations. He re-joined the Minister of Foreign Affairs in 1948 and was posted in London. In 1959, he was appointed Assistant Under-Secretary of State for External Affairs. He served as Deputy Under Secretary of State for External Affairs from August 1964 until July 1966. From 1966 to 1970, he was the Canadian ambassador to the United States. From 1970 to 1974, he held the top job in Canada's foreign service as Under Secretary of State for External Affairs. From 1976 to 1980, he was the Canadian ambassador to Ireland.
In 1973 he received the Government of Canada Public Service Outstanding Achievement Award. In 1975 he was made a Companion of the Order of Canada.
Death
Ritchie died on January 24, 2002, in Ottawa, Ontario.