WASHINGTON — President-elect Joseph R. Biden Jr. has worked with the former aide he wants to be secretary of state since their time at the Senate Foreign Relations Committee in the 1990s. His nominee for agriculture secretary endorsed his first presidential bid more than 30 years ago. And he knows his choice for Pentagon chief from the retired general’s time in Iraq, where Mr. Biden’s son Beau, a military lawyer, also served on the general’s staff.

For all the talk that Mr. Biden is abiding by a complicated formula of ethnicity, gender and experience as he builds his administration — and he is — perhaps the most important criteria for landing a cabinet post or a top White House job appears to be having a longstanding relationship with the president-elect himself.

His chief of staff, Ron Klain, goes back with him to the days of Anita Hill and Clarence Thomas when Mr. Biden was the chairman of the Senate Judiciary Committee and Mr. Klain was on his staff. John Kerry, his climate envoy, is an old Senate buddy. Even Vice President-elect Kamala Harris, who is not a longtime confidante and ran an aggressive campaign against Mr. Biden, had a close relationship with Beau Biden before he died — a personal credential that is like gold with the man about to move into the Oval Office.

In accepting Mr. Biden’s nomination to be the first Black man to run the Defense Department, Gen. Lloyd J. Austin III on Wednesday called Beau a “great American” and recalled the time he spent with him in Iraq, and their conversations after he returned home, before his death from a brain tumor in 2015.

“As you, too, can attest, madam vice president-elect, Beau was a very special person and a true patriot, and a good friend to all who knew him,” General Austin said.

It is a sharp contrast to President Trump, who assembled a dysfunctional collection of cabinet members he barely knew and after an initial honeymoon spent their time constantly at risk of being fired. With nearly half of Mr. Biden’s cabinet and many key White House jobs announced, his administration looks more like a close-knit family.

But there are risks in Mr. Biden’s approach, which departs sharply from Abraham Lincoln’s famous desire for a “team of rivals” in his cabinet who could challenge one another — and the president. And while every president brings in a coterie of longtime advisers, few have had the longevity of Mr. Biden’s nearly five decades in Washington, and prized so much the relationships he developed along the way.

Relying on advisers and cabinet officials steeped in old Washington — and Mr. Biden’s own worldview — lends an air of insularity to his still-forming presidency at a time when many Americans are expecting fresh ideas to confront a world that is very different from the one that the president-elect and his friends got to know when they were younger.

Even some allies in the Democratic Party say they worry that Mr. Biden’s reliance on the same people threatens to undermine his ability to find solutions to the country’s problems that go beyond the usual ones embraced by the establishment in Washington.

Representative-elect Mondaire Jones of New York, 33, who will serve as the freshman representative to the House Democratic leadership, praised Mr. Biden’s choices so far as “highly competent” but added that “competency alone is insufficient for purposes of building back better.”

“One risk of Joe Biden nominating or otherwise appointing only people with whom he has close relationships is he may miss the moment,” he said.

Faiz Shakir, who served as Senator Bernie Sanders’s campaign manager and negotiated with the Biden team over the summer as part of a unity task force, said the biggest bias he has seen from the Biden transition team has been in favor of “credentialing” — both in terms of Washington experience, often with the president-elect, and education.

He said he worried the team was leaning “so much on technocratic competence based on credentialing that it misses the opportunity to introduce fresh blood and new thinking more closely associated with the struggles of the working class.”

And Representative Adriano Espaillat, Democrat of New York, urged Mr. Biden to embrace “a little bit more competitiveness inside” a team that so far appears mostly like-minded. Tackling the big problems in American in the wake of the pandemic “is going to require a lively debate,” Mr. Espaillat said. “It doesn’t have to be a room full of people you like.”

But Mr. Biden has not been shy about describing what is important to him as he builds his team.

“I’ve seen him in action,” Mr. Biden said of Antony J. Blinken, his incoming secretary of state and a longtime adviser.

“I’ve worked with her for over a decade,” Mr. Biden said of his new director of national intelligence, Avril D. Haines.

“One of my closest friends,” Mr. Biden hailed Mr. Kerry when he announced the former secretary of state’s new climate role.

And in an article published in The Atlantic on Tuesday, the president-elect explained one of the key reasons he chose General Austin.

“I’ve spent countless hours with him, in the field and in the White House Situation Room,” Mr. Biden wrote. “I’ve sought his advice, seen his command, and admired his calm and his character.”

Those who know Mr. Biden say he is confident of his own ability as a judge of character and has leaned on some of the same team of counselors for decades. His longtime Senate chief of staff and brief successor in the Senate, Ted Kaufman, is helping to lead the transition. Among his top incoming White House advisers, his counselor, Steve Ricchetti, and senior adviser, Mike Donilon, are longtime loyalists.

Other aides are reprising roles they held in Mr. Biden’s vice-presidential office — only now at the White House itself. Jake Sullivan, the national security adviser, held that post for Mr. Biden, and Jared Bernstein, who was an economic adviser, is now a member of the Council of Economic Advisers.

“He’s got this wonderful team — not of rivals but of talented people that he’s either worked with or observed over the years,” said Joseph Riley, the former mayor of Charleston, S.C., and a man Mr. Biden once called “America’s mayor.”

“He has amassed a collection of talented people who he has watched, listened to, leaned on over the years, and he is a quick study,” Mr. Riley said.

Not every appointee is a Biden intimate. This week, Mr. Biden rolled out his health care team and badly bungled the name of his incoming secretary of health and human services — Xavier Becerra — before correcting himself.

Turning to people close to him to run with long experience in government may be an advantage during confirmation battles in the deeply divided Senate. Many of his picks — like Tom Vilsack, who served for eight years as secretary of agriculture under President Barack Obama and has been nominated for the same job again — are well known to Republicans.

“I think he did an outstanding job for eight years and he’ll do an outstanding job for no more than four years,” Senator Charles E. Grassley, Republican of Iowa and the chairman of the Senate Finance Committee, told reporters when asked about Mr. Biden’s decision to nominate Mr. Vilsack.

But a bigger test for Mr. Biden will be his decision on who should be attorney general and run the Justice Department at a time when racial tensions have roiled the country.

On Tuesday, a group of activists met with Mr. Biden to press him on nominating a Black person who will focus on civil rights and social justice issues. But with an African-American now ready to lead the Defense Department — ensuring that the State, Treasury, Justice and Defense Departments will not all be led by white people — a number of prominent Democrats believe the president-elect may turn to Senator Doug Jones of Alabama, who is white.

Mr. Jones would most likely prove easy to confirm in a closely divided Senate given his warm relationships with senators in both parties, including Alabama’s senior senator, Richard C. Shelby, a Republican.

But Mr. Jones has something else working in his favor: a long history with Mr. Biden.

As a young law student in Birmingham, Ala., Mr. Jones was wowed by a visit from a freshman senator from Delaware and introduced himself to Mr. Biden. They grew closer when Mr. Jones moved to Washington to work on the Senate Judiciary Committee. And in 1987, Mr. Jones served as Alabama co-chair on Mr. Biden’s first campaign for president.

Jonathan Martin and Emily Cochrane contributed reporting.