Inouye, a senator since January 1963, was currently the longest serving senator and was president pro tempore of the Senate, third in the line presidential succession. His office said Monday that he died of respiratory complications at a Washington-area hospital.
Less than an hour after Inouye's passing, Senate Majority Leader Harry Reid announced Inouye's death to a stunned chamber. "Our friend Daniel Inouye has died," Reid said somberly. Shocked members of the Senate stood in the aisles or slumped in their chairs.
Inouye was a World War II hero and Medal of Honor winner who lost an arm to a German hand grenade during a battle in Italy. He became the first Japanese-American to serve in Congress, when he was elected to the House in 1959, the year Hawaii became a state. He won election to the Senate three years later and served there longer than anyone in American history except Robert Byrd of West Virginia, who died in 2010 after 51 years in the Senate.
President Barack Obama, a native of Hawaii, said in a statement, "Tonight, our country has lost a true American hero with the passing of Sen. Daniel Inouye. ... It was his incredible bravery during World War II - including one heroic effort that cost him his arm but earned him the Medal of Honor - that made Danny not just a colleague and a mentor, but someone revered by all of us lucky enough to know him."
Inouye died after a relatively brief hospitalization. Once a regular smoker, he had a portion of a lung removed in the 1960s after a misdiagnosis for cancer. Just last week, he issued a statement expressing optimism about his recovery.
Despite his age and illness, Inouye's death shocked members of the Senate.
"I'm too broken up," said Sen. Patrick Leahy, D-Vt., who becomes president pro tem of the Senate. Leahy also is poised to take over the Senate Appropriations Committee.
"He was the kind of man, in short, that America has always been grateful to have, especially in her darkest hours, men who lead by example and who expect nothing in return," said Senate Minority Leader Mitch McConnell, R-Ky.
Hawaii Gov. Neil Abercrombie will appoint a replacement, choosing from a list of three candidates selected by the state Democratic Party. "We're preparing to say goodbye," Abercrombie said. "Everything else will take place in good time."
Inouye has represented Hawaii since it became a state in 1959, first in the House. He was handily re-elected to a ninth term in 2010 with 75 percent of the vote.
His last utterance, his office said, was "Aloha."
Inouye became president pro tem of the Senate in 2010, a largely ceremonial post that also placed him in the line of succession to the presidency, after the vice president and the speaker of the House.
Earlier, he had taken the helm of the powerful Appropriations Committee, where he spent most of his Senate career attending to Hawaii. At the height of his power, Inouye routinely secured tens of millions of dollars annually for the state's roads, schools, national lands and military bases.
Although tremendously popular in his home state, Inouye actively avoided the national spotlight until he was thrust into it. He was the keynote speaker at the 1968 Democratic National Convention, and later reluctantly joined the Senate's select committee on the Watergate scandal. The panel's investigation led to the resignation of President Richard Nixon.
Inouye also served as chairman of the committee that investigated the Iran-Contra arms and money affair, which rocked Ronald Reagan's presidency.
A quiet but powerful lawmaker, Inouye ran for Senate majority leader several times without success. He gained power as a member of the Senate Appropriations Committee and chairman of the defense appropriations subcommittee before Republicans took control of the Senate in 1994.
When the Democrats regained control in the 2006 elections, Inouye became chairman of the Senate Commerce Committee. He left that post two years later to become chairman of the powerful Senate Appropriations Committee.
Inouye also chaired the Senate Indian Affairs Committee for many years. He was made an honorary member of the Navajo nation and given the name "The Leader Who Has Returned With a Plan."
In 2000, Inouye was one of 22 Asian-American World War II veterans who belatedly received the nation's top honor for bravery on the battlefield, the Medal of Honor. The junior senator from Hawaii at the time, Daniel Akaka, had worked for years to get officials to review records to determine if some soldiers had been denied the honor because of racial bias.
Inouye's first political campaign in 1954 helped break the Republican Party's political domination of Hawaii. He was elected to the Territorial House of Representatives, where he served as majority leader. He became a territorial senator in 1958.
Inouye was serving as Hawaii's first congressman in 1962, when he ran for the Senate and won 70 percent of the vote against Republican Benjamin Dillingham II, a member of a prominent Hawaii family.
In 1968, President Lyndon Johnson urged Vice President Hubert Humphrey, who had won the Democratic nomination for president, to select Inouye as his running mate. Johnson told Humphrey that Inouye's World War II injuries would silence Humphrey's critics on the Vietnam War.
"He answers Vietnam with that empty sleeve. He answers your problems with (Republican presidential candidate Richard) Nixon with that empty sleeve," Johnson said.
But Inouye was not interested.
"He was content in his position as a U.S. senator representing Hawaii," Jennifer Sabas, Inouye's Hawaii chief of staff, said in 2008.
Inouye reluctantly joined the Watergate proceedings at the strong urging of Senate Democratic leader Mike Mansfield. The panel's investigation of the role of the Nixon White House in covering up a burglary at Democratic National Committee headquarters at the Watergate in June 1972 ultimately prompted the House to initiate impeachment proceedings against Nixon, who resigned before the issue reached a vote in the House.
In one of the most memorable exchanges of the Watergate proceedings, an attorney for two of Nixon's closest advisers, John Ehrlichman and Bob Haldeman, referred to Inouye as a "little Jap."
The attorney, John J. Wilson, later apologized. Inouye accepted the apology, noting that the slur came after he had muttered "what a liar" into a microphone that he thought had been turned off following Ehrlichman's testimony.
After the hearings, Inouye said he thought the committee's findings "will have a lasting effect on future presidents and their advisers. It will help reform the campaign practices of the nation."
He achieved celebrity status when he served as chairman of the congressional panel investigating the Iran-Contra affair in 1987. That committee held lengthy hearings into allegations that top Reagan administration officials had facilitated the sale of weapons to Iran, in violation of a congressional arms embargo, in hopes of winning the release of American hostages in Iran and to raise money to help support anti-communist fighters in Nicaragua.
"This was not a happy chore, but it had to be done," Inouye said of the hearings.
The panel sharply criticized Reagan for what it considered laxity in handling his duties as president. "We were fair," Inouye said. "Not because we wanted to be fair but because we had to be fair."
Born Sept. 7, 1924, to immigrant parents in Honolulu, Inouye was 17 and dreaming of becoming a surgeon when Japanese planes flew over his home to bomb Pearl Harbor on Dec. 7, 1941, changing the course of his life.
In 1943, Inouye volunteered for the Army and was assigned to the famed Japanese-American 442nd Regimental Combat Team, which earned the nickname "Go For Broke" and was one of the most decorated units of the war. Inouye rose to the rank of captain and earned the Distinguished Service Cross and Bronze Star. Many of the 22 veterans who received Medals of Honor in 2000 had been in the 442nd.
Unlike the families of many of his comrades in arms, Inouye's wasn't subjected to the trauma and indignity of being sent by the U.S. government during the war to internment camps for Japanese Americans.
"It was the ultimate of patriotism," Inouye said at a 442nd reunion. "These men, who came from behind barbed wire internment camps where the Japanese-Americans were held, to volunteer to fight and give their lives. ... We knew we were expendable."
Inouye said he didn't feel he had any choice but to go to war.
"I tried to put myself in the shoes of my neighbors who were not Japanese," Inouye once said. "I felt that there was a need for us to demonstrate that we're just as good as anybody else.
"The price was bloody and expensive, but I felt we succeeded," he said.
Inouye's dream of becoming a surgeon ended in the closing days of the war.
On April 21, 1945, he was leading a charge on a machine gun nest in Italy's Po Valley. He was shot in the abdomen, but kept inching toward the machine gun and managed to throw two grenades before his right arm was shattered by a German grenade. Even then, he continued to direct his platoon.
"By his gallant, aggressive tactics and by his indomitable leadership, Second Lieutenant Inouye enabled his platoon to advance through formidable resistance," his Medal of Honor citation said.
He spent the next 20 months in military hospitals. During his convalescence, Inouye met Bob Dole, the future majority leader of the Senate and 1996 Republican presidential candidate, who also was recovering from severe war injuries. The two later served together in the Senate for decades.
Despite his military service and honors, Inouye returned to an often-hostile America. On his way home from the war, he often recounted, he entered a San Francisco barbershop only to be told, "We don't cut Jap hair."
He returned to Hawaii and received a bachelor's degree in government and economics from the University of Hawaii in 1950. He graduated from George Washington University's law school in 1952.
Inouye proposed to Margaret Shinobu Awamura on their second date, and they married in 1949. Their only child, Daniel Jr., was born in 1964. When his wife died in 2006, Inouye said, "It was a most special blessing to have had Maggie in my life for 58 years."
He remarried in 2008, to Irene Hirano, a Los Angeles community leader.
Inouye shunned the trappings of Washington's elite, rising to go to work in a car pool and leaving the telephone number of his Bethesda, Md., home in the phone book.
He took pride in handling even the smallest requests from his constituents.
He said he once was awakened at 2 a.m. by a telephone call from a Hawaii family asking for help in getting a soldier home for a family emergency. Inouye said he immediately called the Pentagon, and 30 minutes later the soldier had his orders to return home.
His staff released the following statement:
United States Senator Daniel K. Inouye, World War II veteran, Medal of Honor recipient and Hawaii's senior Senator, passed away from respiratory complications at 5:01 p.m. Eastern Standard Time today at the Walter Reed National Military Medical Center.
His wife Irene and his son Ken were at his side. Last rites were performed by Senate Chaplain Dr. Barry Black.
He is survived by his wife, Irene Hirano Inouye, his son Daniel Ken Inouye Jr., Ken's wife Jessica, and granddaughter Maggie and step-daughter Jennifer Hirano. He was preceded in death his first wife, Maggie Awamura.
Senator Inouye's family would like to thank the doctors, nurses and staff at Walter Reed National Military Medical Center for the extraordinary care he received.
The story of Dan Inouye is the story of modern Hawaii. During his eight decades of public service, Dan Inouye helped build and shape Hawaii.
Senator Inouye began his career in public service at the age of 17 when he enlisted in the U.S. Army shortly after Imperial Japan attacked Pearl Harbor on December 7, 1941. He served with 'E' company of the 442 Regimental Combat Team, a group consisting entirely of Americans of Japanese ancestry. Senator Inouye lost his arm charging a series of machine gun nests on a hill in San Terenzo, Italy on April 21, 1945. His actions during that battle earned him the Medal of Honor.
Following the war he returned to Hawaii and married Margaret "Maggie" Awamura, and graduated from the University of Hawaii and the George Washington University School of Law.
After receiving his law degree, Dan Inouye, returned to Hawaii and worked as a Deputy Prosecuting Attorney for the City and County of Honolulu. He recognized the social and racial inequities of post-war Hawaii, and in 1954 was part of a Democratic revolution that took control of the Territorial Legislature.
Following statehood in 1959, Dan Inouye was privileged to serve as Hawaii's first Congressman. He ran for the Senate in 1962 where he served for nearly nine consecutive terms.
Dan Inouye spent his career building an enduring federal presence in Hawaii to ensure that the state would receive its fair share of federal resources. He worked to expand the military's presence on all major islands, stabilizing Pearl Harbor, building up the Pacific Missile Range and constructing a headquarters for the United States Pacific Command.
He has worked to build critical roads, expanded bus services statewide and secured the federal funds for the Honolulu Rail Transit project. He championed the indigenous rights of Native Hawaiians and the return of Kahoolawe.
He fought for the rights and benefits for veterans. Senator Inouye has left an indelible mark at the University of Hawaii, including support for major facilities and research assets. He has long supported local agriculture and alternative energy initiatives.
Dan Inouye was always among the first to speak out against injustice whether interned Japanese Americans, Filipino World War II veterans, Native Americans and Native Hawaiians.
A prominent player on the national stage, Senator Inouye served as Chairman of the Senate Committee on Appropriations, the Senate Commerce Committee and was the first Chairman of the Senate Select Committee on Intelligence.
After developing a reputation as a bipartisan workhorse, who always would put country above party, he was asked by the Senate leadership to chair the special committee investigating the Iran Contra Affair. This was after a successful tenure as a member of the Watergate Committee.
When asked in recent days how he wanted to be remembered, Dan said, very simply, "I represented the people of Hawaii and this nation honestly and to the best of my ability. I think I did OK."
His last words were, "Aloha."