A third-generation Japanese American, Matsui was born in Sacramento, California, and was six months old when he and his family were taken from Sacramento and interned by the U.S. government at the Tule Lake War Relocation Center in 1942.
Matsui graduated from the University of California, Berkeley, in 1963 with a BA in political science, and then graduated from Hastings College of Law in 1966. He founded his own Sacramento law practice in 1967.
In 1971 Matsui was elected to the Sacramento City Council. He won re-election in 1975 and became vice mayor of the city in 1977. In 1978, Matsui ran for the Democratic nomination in what was then the 3rd District after 12-term incumbent John E. Moss announced his retirement. He won a five-way Democratic primary with 36 percent of the vote, besting a field that included State Assemblyman Eugene Gualco and Sacramento Mayor Phil Isenberg. He then defeated Republican Sandy Smolley with 53 percent of the vote. He would never face another contest nearly that close in what has long been the most Democratic district in interior California, and would be reelected 13 times. After his initial contest, he never dropped below 68 percent of the vote. He was reelected in 1982 with no major-party opposition, and was unopposed in 1984. His district was renumbered as the 5th District after the 1990 Census.
In 1988, Matsui succeeded in helping pass the Civil Liberties Act of 1988, which produced an official apology from the Federal government for the World War II internment program and offered token compensation to victims. He was also instrumental in the designation of Manzanar internment camp as a national historic site and in obtaining land in Washington, D.C. for the memorial to Japanese-American patriotism in World War II.
He was a chairman of the Democratic Congressional Campaign Committee, ranking member of the Ways and Means Subcommittee on Social Security, and third-ranking Democrat on the Ways and Means Committee. During his term he was noted for his staunch opposition to privatization of Social Security. He had a mostly liberal voting record having opposed the Defense of Marriage Act, the ban on partial-birth abortions, and the Private Securities and Litigations Reform Act.
In what would be his last election, 2004, he faced Republican Mike Dugas and easily won a 14th term with 71.4% of the vote, compared to Dugas' 23.4%. Green Party opponent Pat Driscoll and John Reiger of the Peace and Freedom Party won 3.4% and 1.8% of the vote, respectively. (DCCC chairs are chosen in part because they are not expected to face serious competition for re-election.)
He was married to the former Doris Okada who, until December 1998, worked as deputy assistant to the President and Deputy Director of Public Liaison for President Bill Clinton, leaving to become senior advisor and director of government relations at the firm of Collier Shannon Scott, PLLC before winning election to her late husband's seat. The Matsuis had one son, Brian, who received his undergraduate and law degrees from Stanford University.
Matsui entered Bethesda Naval Hospital on December 24, 2004 with pneumonia. It was a complication from Myelodysplastic syndrome, a rare stem cell disorder that causes an inability of the bone marrow to produce blood products, such as red blood cells, white blood cells and platelets. He died of pneumonia on January 1, 2005.
In the special election held on March 8, 2005 to fill the vacant 5th Congressional District seat, Matsui's widow Doris won with more than 68 percent of the vote. She was sworn in on March 10, 2005.