At 9 Monday morning, Peter Shapiro, the new county executive of Essex County, N.J., was on his second breakfast meeting here -- with one more still to go.

Five years ago, he would not have been awake at that hour: He was a senior at Harvard, waking up in time to eat lunch and maybe catch an afternoon class. He'd spend the rest of the day and night working at the Harvard Crimson where he was managing editor, then unwind in the wee hours by playing pinball.

Now he makes $50,000 a year in what some observers say is the second most powerful political job in New Jersey, after the governor.

Shapiro is one of a handful of under-30 Wunderkinder who have won substantial political offices in the '70s. Rep.James Michael Shannon (D-Mass.), who won now-Sen. Paul Tsongas' seat, is the youngest member of the House at the age of 26. Rep. Tom Downey (D-N. Y.) made it to the House four years ago at the age of 25.

A liberal Democrat, Shapiro broke the Democratic Party's primary opposition to win a newly created office last November as the first county executive of the largest county in New Jersey -- roughly a million people, including the poor of Newark and the rich of Short Hills.

He immediately inherited coutrol of 10,000 patronage jobs, got a phone call from President Carter offering help, and became something of a celebrity. Splashes of stories in the New York and New Jersey papers chronicled his rapid rise to the top and his charm as a young bachelor.

All this at the age of 26.

"It's pretty wild," he said, slouching in a chair at the Madison Coffee Shop, feet stretched out. "In the state legislature you can concentrate on a few issues. As county executive you have to have a position on virtually everything."

Shapiro does. After only four months in office, he's gotten a $209-million budget passed (a few million short of what he wanted), established a good relationship with the mayor of Newark (a city Shapiro would particularly like to focus on) and begun to forge good supportive relationships among the nine freeholders who are the Essex equivalent of a county council -- even those who did not support him during the election. He is insistent about developing job skills for the high numbers of jobless in the county, and advocates more production of solar energy.

Shapiro has a reputation for jumping in fast. When elected to the state assembly at the age of 23, he grabbed a position on the prestigious appropriations committee which reviews the state budgets. When the county Democratic Party boss told the young man how to use $15,000 allotted to him for staff, Shapiro ignored him.

Instead of hiring the people he was supposed to, "I took out a classified ad for one of the positions on the staff," Shapiro said. "We interviewed about 50 people and hired the best. That's what I got elected to do -- to take on the machine."

In his first race for state assembly, a boyish and handsome Shapiro won largely by personally knocking on thousands of doors -- although at the end of the campaign, he got a little help from his college friend Bobby Kennedy, son of the late Robert F. Kennedy.

Over a breakfast of club soda Monday, Shapiro, who was here this week for the annual National Association of Counties (NACo) conference, yawned from time to time as he talked easily about himself and his office -- one minute quoting budget figures, and the next poking fun.

"We've hired a ton of women," he said about the county government. "We're trying to change the look of the place." He smiled.

Shapiro became interested in politics in the summer of 1973, between his junior and senior years in college. He worked for The Wall Street Journal and became dismayed with the people in Congress. After college he worked for one of New Jersey Gov. Brendan Byrne's cabinet members. "I decided that this government could use some good people," Shapiro said, and then smiled at the pretentiousness of his remark.

As a measure of his new-found responsibility, Shapiro was booked solid into meetings with Carter staff aides these past three days, spending more time on Capitol Hill than at the NACo conferences. He met with Ann Wexler, Jack Watson, Tim Kraft -- all wanting to know how they could help.

But even his intermittent presence at the conference was noted. Another young county executive -- who, when first elected at the age of 30, claimed to be the youngest in the county -- ran up to Shapiro and exclaimed, "You stole my title!"

In addition to Hill people, Shapiro hobnobbed with like-minded progressives from the local Conference on Alternative State and Local Public Policies. They threw him a reception -- and their encouragement.

Now that Shapiro makes a county executive's salary, he plans to buy a house and move out of his small apartment in Irvington, which has a large low-income population.

"Why did I live there?" he said. "I was lower income!"