In 1905, Yoshizawa married the eldest daughter of politician (and future Prime Minister) Tsuyoshi Inukai, and moved to London. He continued to live in England for the next several years, eventually becoming First Secretary to the Japanese embassy. He was given the post of Consul-General in Hankou, China in 1912.
In the period immediately prior to the start of the Pacific War, Yoshizawa was appointed as a special envoy by Prime Minister Fumimaro Konoe to the Netherlands East Indies following the diplomatic mission of Ichizo Kobayashi. Yoshizawa was assigned to present a new set of demands on the Dutch government in Batavia, which were deliberately intended to be unacceptable.[4]
Adherence to Japan's vision and policy in South East Asia,
Unrestricted rights to explore and exploit minerals all over the Dutch East Indies
Unrestricted fishing and shipping rights in all the waters of the Dutch East Indies
Unrestricted rights to start all sorts of commercial enterprises
Japans export to the Dutch East Indies must be increased to more than 80% of all imports of the Dutch East Indies
The existing demand for oil was slightly increased to 3,800,000 tons
The Dutch East Indies was to supply Japan with 1,000,000 tons of tin, 400,000 tons of bauxite, 180,000 tons of nickel, 30,000 tons of rubber, 30,000 tons of coconut oil and 10,000 tons of sugar.
Airline and telegraph connections between Japan and the Dutch East Indies
In December 1940, Yoshizawa was met by Hubertus Johannes van Mook, deputy minister of Economic Affairs, K. L. J. Enthoven, director of Justice, and Hoessein Djajadiningrat, director of Education and Religion. The negotiations dragged on unsuccessfully, and on 11 June 1941, the Liaison Meeting of the Imperial General Headquarters and Government decided to recall Yoshizawa and terminate the talks.[5] Yoshizawa suddenly announced to the Dutch his plan to depart and asked to be received by the Dutch Governor-General A. W. L. Tjarda van Starkenborgh Stachouwer on 17 June 1941. The latter was worried that he might receive a declaration of war, but to his relief Yoshizawa only handed him a draft declaration stating that the negotiations had ended without an agreement.
From 1941–1944, Yoshizawa served as Japanese ambassador to French Indochina. The posting was mostly symbolic, as by then mostly under Japanese military occupation. In August 1945, he became a member of the Privy Council.