He held several high appointments in the foreign service, most notably as U.S. minister to Switzerland throughout World War II.

After law school, Harrison became the private secretary of United States Ambassador to Japan Thomas J. O'Brien.

The two men picked Leland Harrison "to take charge of the collection and examination of all information of a secret nature coming into the Department from various sources and also to direct the work of the agents specially employed for that purpose.

A short time later, however, he returned to government service as chief of the International Relations Division of the United States Tariff Commission.

He endorsed many of these reports as being credible and recommended action be taken to assist in the relief and rescue of Jews in Nazi-controlled territories.