Peter Shapiro, who as a 23-year-old insurgent was the youngest person ever elected to the New Jersey General Assembly and who later became the first Essex County executive, died on Thursday at his home in South Orange, N.J. He was 71.

The cause was respiratory failure after long being treated for lung disease, his wife, Bryna Linett, said.

As a young assemblyman, Mr. Shapiro helped streamline the way local government worked after successfully campaigning in 1977 for a charter change that coupled Essex County’s nine-member Board of Chosen Freeholders (now the Board of County Commissioners) with a strong county executive in what was the state’s most populous county, which includes Newark.

He ran for the newly created position the next year, defeating a Democratic organization candidate for the nomination and overpowering a Republican rival, Robert F. Notte, by a record margin. As county executive, he reformed the county’s welfare program, decentralized other services to make them more responsive to localities, refinanced the pension system and lowered the county property tax rate.

“Peter, what you did for Essex County is precisely what I am attempting at the state level,” Gov. Thomas H. Kean, a Republican, said at the time.

Seeking re-election in 1982, and after defeating two rivals in a Democratic primary, Mr. Shapiro said: “We were able to show that it’s possible to take an old urban government like Essex County’s, a government that a lot of people had given up on, and make it more responsive, more efficient, bring down the taxes and make it a model of what’s right with government.”

After cruising to re-election in a landslide, Mr. Shapiro concluded that he could replicate his success in Essex as governor. In 1985, he challenged Mr. Kean, who had entered office on shaky ground during a recession. But by then, the state’s economy was booming again, and Mr. Shapiro lost the race, 71 percent to 24 percent, the largest margin in a New Jersey governor’s race.

In 1986, after his fellow Democratic state legislators voted in favor of higher spending and taxes than Mr. Shapiro had recommended, he was defeated for re-election as county executive by Nicholas R. Amato, a former Democrat.

Peter Ian Shapiro was born on April 18, 1952, in Newark to Dr. Myron and Henrietta (Asch) Shapiro. His father was an ear, nose and throat surgeon and a professor at the New Jersey College of Medicine and Dentistry (now the University of Medicine and Dentistry of New Jersey). His mother ran a bookstore in South Orange and managed the household. Peter grew up in Orange and South Orange.

At Columbia High School in Maplewood, he was expelled for leading a protest against the Vietnam War but was reinstated after the American Civil Liberties Union intervened.

After graduating, he traveled as a merchant seaman, then earned a bachelor’s degree in economics and history from Harvard College in 1974. After working for Brendan T. Byrne’s campaign for governor, he was hired as an aide by a family friend, Alan Sagner, the new state transportation commissioner.

In his first Assembly race, barely a year after graduating from college, Mr. Shapiro campaigned door to door to defeat the organization candidate, Rocco Neri, in the Democratic primary, winning by 183 votes of 8,530 cast. He served in the Assembly from 1976 to 1979, when he took office as county executive.

He married Ms. Linett, a teacher, in 1982. In addition to her, he is survived by their son, Samuel, and two sisters, Nancy and Margaret (who goes by Pooh) Shapiro.

After leaving office, Mr. Shapiro worked for Citibank and later founded Swap Financial Group, a hugely successful independent investment adviser based in Manhattan. He also counseled other firms and government agencies on regulatory reform and how to extricate themselves from the aftermath of the 2008 financial crisis. He retired in 2019.

Sofia Poznansky contributed reporting.