In 1925, as diplomatic relations between the Soviet and the Japanese governments were established, was appointed Secretary of the Embassy in Moscow. Served as Counselor of the Embassy in Moscow from June 1939 to August 1940.
Nishi served as Japanese Vice-Minister for Foreign Affairs in 1941, at the time of the attack on Pearl Harbor. Available evidence shows that he was not notified in advance about the intention to launch the attack, and on the morning of December 8, 1941, heard about it over the radio. He resigned his position as vice-minister on September 2, 1942, following the resignation of Foreign Minister Shigenori Togo.
After the war
During the Tokyo War Crimes Trial at the International Military Tribunal for the Far East in 1946-1948, he served as defence counsel to some of the Japanese war criminals. He served as first postwar Japanese ambassador to Australia in 1952-1955. As ambassador, he attended the memorial service for ANZAC in April 1953, and no protests were made against his presence.
He served as Ambassador to the UK in 1955-1957.
- "Another Jap Minister Resigns" The Argus, September 3, 1942 (Australian newspaper appearing until 1954)
- Takeo Iguchi, "Covering their tracks on the way to war" The Japan Times, Sunday, Dec. 15, 2002  (the author of this article served under Nishi in later years)
- Controversy over Japan's Surprise Attack on the United States and Britain in December 1941, by Prof. Takeo Iguchi
- New York Times, September 3, 1942
- Canberra Times, April 27, 1953
- Vivienne Kenrick, article about Eiko Todo, The Japan Times, Nov. 10, 2001