Sonoda joined the Japanese army in 1938, and served both in China and in the Pacific area during World War II. More specifically, he was commander of a kamikaze squad during the war. In 1947, Sonoda was elected to the House of Representatives, the lower house of the Diet, being a member of lower house for Kumamoto Prefecture.
In the 1950s, he was special envoy of the LDP. He served as parliamentary vice-foreign minister in 1955, and actively involved in normalizing the relations between Japan and the USSR. However, in 1960, Sonoda resigned from the LDP due to his objections to the ratification of the US-Japan mutual security treaty.
After rejoining the LDP, Sonoda also served as vice speaker of the lower house for two terms. He served as minister of health and welfare, then he held this position again. In addition, Sonoda was chief cabinet secretary in the cabinet led by Takeo Fukuda.
Within the LDP Sonoda was against the Nakasone faction and formed his own. He and the members of his faction joined the faction headed by Fukuda in 1972. However, he later left it and joined the faction headed by Masayoshi Ōhira.
Sonoda served as minister of foreign affairs three times: in the cabinet of prime minister Takeo Fukuda , in the cabinet of prime minister Masayoshi Ohira , and in the cabinet of prime minister Zenko Suzuki.
During his first term in the ministry of foreign affairs, Japan signed the treaty of peace and friendship with China. This treaty formed the basis of the relationships between two countries. Sonoda represented his country at the signature of this treaty in Beijing in 1978. Sonoda was secondly appointed foreign minister to the cabinet of Masayoshi Ohira who kept this and other three ministries for his own faction. When in office for the second time, Sonoda visited five African countries in July 1979, including Tanzania, Nigeria, Kenya, Ivory Coast and Senegal. He also traveled South America in August 1979.
On 17 May 1981, Sonoda was appointed by then prime minister and his close friend Zenko Suzuki as foreign minister for the last time due to unexpected resignation of the former foreign minister Masayoshi Ito. Sonoda called for adopting the omnidirectional diplomacy and unlike his two predecessors, issued entry visas to Soviet economic delegations. Sonoda was replaced by Yoshio Sakurauchi who was appointed foreign minister by prime minister Zenko Suzuki on 30 November 1981. The reason for Sonoda's removal from his post was his blunt remarks concerning U.S. policies in June 1981 as well as his other statements detrimental to Japan's relations with South Korea.
He was originally a member of the Democratic Party. Then he became a member of the Liberal Democratic Party (LDP) when the Democratic Party joined the Liberals.
Sunao Sonoda married twice. His son from the first marriage, Hiroyuki Sonoda, ran for his father seat in Kumamoto Prefecture in the general elections of 1986. Sonoda'a second wife, Tenkoko Sonoda, also tried to take over her husband's seat in the same election following his death.Tenkoko Sonoda was a member of the Diet during her marriage to Sunao.They married after World War II and had two children.