Democratic New Jersey Sen. Frank Lautenberg, who at 89 was the oldest member of the Senate, died Monday due to complications from viral pneumonia, his office said.
Lautenberg was the last remaining World War II veteran in the Senate. He died at New York-Presbyterian Hospital.
The Paterson-born lawmaker had announced in February that he wouldn't seek re-election next year.
Lautenberg was first elected to the Senate in 1982 -- which was his first run for public office -- and was re-elected in 1988 and 1994. He briefly retired and then successfully ran for office again in 2002, followed by a re-election in 2008.
Lautenberg was last on the Senate floor April 17, when he received an ovation from his fellow senators as he arrived in a wheelchair to vote for a gun control measure.
He cast his 9,000th vote in December 2011, and at the time, Senate Majority Leader Harry Reid said "Frank Lautenberg has been one of the most productive senators in the history of this country."
Lautenberg, who enlisted in the Army when he was 18, served in Europe during World War II. He founded a company, Automatic Data Processing, and then left to run for office.
Lautenberg is survived by his wife, six children and 13 grandchildren.
Republican Gov. Chris Christie will appoint a senator to fill the seat.