The cause was a lung infection, a son, Tom Byrne, told the Associated Press.
A soft-spoken but scrappy Irish-American Democrat, Mr. Byrne won the 1973 governor's race amid a wave of disdain for the Watergate scandal and leveraged the public's appetite for aggressive government.
Mr. Byrne cultivated a reputation as a no-nonsense, incorruptible public servant who put the interests of his state before himself. He became known as "the man the mob couldn't buy" after an organized-crime figure he prosecuted in the 1960s, Simone "Sam the Plumber" DeCavalcante, called him a "Boy Scout" and added, "You can't get to him. He can't be bought."
The FBI picked up the conversation on a wiretap, and a backhanded compliment morphed into Mr. Byrne's 1973 campaign slogan: "One honest man can make a difference."
Months after running for office saying there was no need to increase taxes, Mr. Byrne created the state's first income tax, making him wildly unpopular and earning him the nickname "one-term Byrne."
The decision came after the New Jersey Supreme Court ruled that the state's method of financing public education through property taxes and other funding mechanisms deprived students of an adequate education.
In what was perhaps a worse offense in New Jersey, Mr. Byrne was seen as boring. During his second year in office, a New York Times article compared him to a dial tone: "steady, dull, predictable."
Notable deaths in 2017
When he invited a delegation of fourth-graders to his Trenton office in 1975, Mr. Byrne was greeted by blank stares when he asked, "Who knows who I am?"
"I wasn't a backslapper or a bon vivant," Mr. Byrne was quoted as saying in a 2014 biography by Donald Linky.
Despite his dry manner and the new income tax, Mr. Byrne had enough political support and savvy to become New Jersey's last Democratic governor elected to two terms. He was the state's oldest living governor.
Well past 90, he remained a well-regarded elder statesman who never lost his sense of humor about his scandal-ridden home state.
"I want to be buried in Hudson County," he said, "so that I can remain active in politics."
Mr. Byrne maintained that his most lasting legacy would be the preservation of the Pinelands, which spans 1.1 million acres in southern New Jersey and makes up 20 percent of the state's land mass.
Mr. Byrne knew little of the Pine Barrens until he read his friend John McPhee's 1968 book of the same name about the rambling oak forests, streams, rivers and vast underground reservoir — an area McPhee said was "headed slowly toward extinction" because of likely development.
Over tennis, Mr. Byrne liked to tell McPhee he would preserve the land and prove the author wrong, according to Linky's biography, "New Jersey Governor Brendan Byrne: The Man Who Couldn't Be Bought."
"Without a doubt, it would have been developed had it not been for him," John Weingart, director of the Eagleton Center on the American Governor at New Jersey's Rutgers University, said in an interview. "It's a rare case in government where it's really about one person."
Brendan Thomas Byrne was born April 1, 1924, into a political family in West Orange, N.J., 20 miles from Manhattan. His father was the local public safety commissioner and president of the Essex County tax board; his grandfather was an alderman.
During World War II, Mr. Byrne served in the Army Air Forces and flew combat missions on B-17s across Europe. He graduated from Princeton University in 1949 on the G.I. Bill and earned a law degree from Harvard University in 1951.
He entered public service in 1955 as an assistant counsel to New Jersey Gov. Robert B. Meyner (D), who quickly promoted him to executive secretary, the equivalent of chief of staff, and four years later named him top prosecutor in Essex County.
During the 1967 Newark riots, Mr. Byrne oversaw the processing of nearly 1,500 arrests and walked the city's streets with a shotgun, according to a 2014 Wall Street Journal article. He went after dishonest construction contractors and organized-crime figures and uncovered scandals in a housing authority and a Newark medical center.
After nine years as prosecutor, Mr. Byrne served a two-year stint as president of the New Jersey public utilities commission before being appointed a state judge in 1970. In one of his most significant decisions, Mr. Byrne ruled the state's death-penalty procedures unconstitutional in 1971.
With his crime-fighting record, and as the Catholic grandson of Irish immigrants, Mr. Byrne cut a Kennedy-esque figure among New Jersey Democrats looking for a gubernatorial candidate in 1973. It didn't hurt that he was tall, with a rugged resemblance to actor Charlton Heston.
Mr. Byrne went on to a landslide victory over Charles W. Sandman Jr., who ousted the scandal-marred sitting governor, William T. Cahill, in the Republican primary.
"My first obligation is to restore integrity in government," Mr. Byrne said after the election.
Soon after taking office, he instituted gasoline rationing during a 1974 shortage caused by an oil embargo. He also launched the country's first emissions-testing program for motor vehicles and successfully called for public financing of the 1977 governor's race and for the creation of a public advocate department.
Mr. Byrne signed a 1976 law that legalized casino gambling in Atlantic City in hopes of revitalizing the shore town and getting mobsters' "filthy hands" off the city. He later said his greatest regret was not creating a mechanism to manage the financial boon to the state.
Initially weakened by his about-face on the income tax, Mr. Byrne tried to turn the decision to his advantage.
"Sometimes you have to admit you were wrong in order to do what's right," he said in a campaign ad.
He employed a crafty strategy of hinting that he might not seek reelection. He encouraged other Democrats to enter the primary, which had the effect of smoothing his path toward the nomination. He delivered strong debate performances and easily beat Republican Raymond Bateman in 1977.
"I knew I'd get reelected," Mr. Byrne said, "when people started waving at me using all five fingers."
When critics accused him of misusing public funds by riding in a state helicopter and by allowing his daughter to use a state car to get to college, he joked, "You have to use the helicopter when your daughter has the car."
As governor, Mr. Byrne pushed for construction of the Meadowlands Sports Complex to entice the New York Giants to cross the Hudson River. (An effort to persuade the football team to add New Jersey to its name fell flat.)
The Brendan Byrne Arena, which was the home to the NBA's New Jersey Nets and the New Jersey Devils of the NHL, opened in 1981 with six sold-out concerts by Bruce Springsteen. It later was renamed for corporate sponsors before it was closed in 2015.
After leaving office, Mr. Byrne practiced law and was sought after as what the New York Times in 2005 called "a professional wise man" and "raconteur."
He and another former governor, Republican Thomas H. Kean, honed an odd-couple shtick that included a regular column in Newark's Star-Ledger newspaper.
Mr. Byrne had seven children with his first wife, the late Jean Featherly. The marriage ended in divorce in 1993, and a daughter died in 2006. He later married public relations executive Ruthi Zinn, who survives, along with six children and nine grandchildren.
"Natural-born politicians walk with a spring in their step, and they can make their face light up with a smile any time they want," Mr. Byrne said in Linky's 2014 biography, explaining his apparent lack of charisma. "But I walked with a kind of shuffle, and I can't smile unless I'm happy. I look around and try to get comfortable but I don't pretend to be delighted if I'm not. I wouldn't know how."
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