After Daniel Inouye was elected to the Senate, Matsunaga succeeded him as the state's sole member of the House of Representatives. After Hawaii was split into districts for the 1970 elections, Matsunaga was elected for , comprising Honolulu's inner ring, and held that seat until 1976. That year, with Hiram Fong retiring, Matsunaga defeated Hawaii's other House representative, Patsy Mink, for the Democratic Party nomination for Senator. Matsunaga then defeated former Republican governor William Quinn in the general election and went on to serve in the United States Senate from 1977 until his death in 1990.