Inouye served in Hawaii's congressional delegation since 1959, when Hawaii became a state. He was elected to the Senate in 1962, making him the most senior senator and the second longest serving in U.S. history, after West Virginia's Robert Byrd.
While Inouye's career in the Senate is remarkable for its duration, his career prior to entering Congress may have been more remarkable, as Jake McIntyre explained in 2008:
Dan Inouye is one of the greatest living Americans. [...] Inouye is a bona fide war hero—a Congressional Medal of Honor recipient—who lost his arm fighting for a nation which was simultaneously interning thousands of his fellow Japanese-Americans. He's a son of immigrants, who after the war joined with a number of other Nisei veterans to found Central Pacific Bank—a venture which helped provide capital to scores of Japanese-Americans in Hawai'i unable to obtain loans from other sources. And he was the first Japanese-American elected to the US House of Representatives, and subsequently the first Japanese-American elected to the Senate.Inouye's Senate career wasn't perfect, but he was for more than 50 years a strong voice for Hawaii and for the Democratic party. Condolences to his family, friends, and state.