Last Friday, when Jared Kushner heard that Hamas would begin talks to release Israeli hostages, he was fielding calls at his mansion, which sits on a man-made island just north of Miami. He jumped into his car and drove the 20 minutes to another mansion — this one owned by the billionaire Steve Witkoff, President Trump’s Middle East peace envoy.

In those crucial moments, the Trump administration’s diplomatic power center was not in Washington but in one of the wealthiest enclaves in Florida.

The two real estate developers, charged with closing the deal on a prime piece of Mr. Trump’s foreign policy ambitions, got to work setting up a command center, where they made and fielded calls from stakeholders, including an impatient president and cabinet members in the Israeli government.

The stage had been set for a peace deal earlier that week, when Prime Minister Benjamin Netanyahu had agreed with Mr. Trump on a 20-point proposal for pursuing a peace agreement.

With terms that largely favored Israel, it was still unknown whether Hamas would sign on and agree to release hostages or relinquish control of the enclave. As the deadline for Hamas to respond ticked closer, Mr. Trump warned Hamas fighters that thousands of them had already been killed, and that many more would be if they did not agree to a deal.

Hamas said hours later that it would begin talks to release hostages.

While he was working alongside Mr. Witkoff in Miami, Mr. Kushner’s advice to the Israelis was not to worry about the rest of the militant group’s statement, which did little to assuage Israeli concerns that Hamas would refuse to give up its arms or political control of Gaza. Mr. Kushner was focused on the first part of the statement, which meant hostages could soon come home.

“Steve and I said to Israel, ‘We encourage you to be positive as well,’” Mr. Kushner recalled in an interview. In phone calls, the Israelis had been telling them that Hamas would outright reject any deal. “‘This is a time to be positive,’” Mr. Kushner reiterated.

Hours later, the office of Prime Minister Benjamin Netanyahu of Israel said that it would agree to begin carrying out the first phase of Mr. Trump’s peace plan.

In his quest to end the Israel-Hamas war, Mr. Trump has not turned to longtime diplomats to get the job done. His advisers say he is not steeped in the details of what a long-term peace agreement would look like. Instead, he has relied on his son-in-law, Mr. Kushner, to step in and add momentum to the negotiations, which Mr. Witkoff had been pursuing for months.

‘You’re Always Negotiating’

Mr. Kushner, 44, had built diplomatic relationships in Arab countries while working as an adviser during Mr. Trump’s first term. He became a key architect of the Abraham Accords, a set of diplomatic agreements that normalized relations between Israel and three Arab states, which gave him an understanding of the complexities of the region and the key players who operate within it.

Mr. Kushner traveled to Egypt on Tuesday alongside Mr. Witkoff and found success. The pair joined a set of mediators who had already been working there for days to convince Hamas to disarm and to give over Israeli hostages who were taken in the wake of terrorist attacks two years ago.

The two spent the plane ride strategizing over ways they felt the deal could fall apart, and what they could do to salvage it. When the two men are working, Mr. Kushner is often drafting the plans as Mr. Witkoff works the phones.

“In New York real estate, you’re always negotiating back and forth,” he said. “But there’s a lot to be negotiated before you get the contract and you put money up hard. I think we’re just used to complex deals that are very dynamic, and with complex characters as well.”

Hours after Mr. Kushner arrived in Egypt, Mr. Trump announced that Israel and Hamas had reached an agreement that could spell the end of a two-year conflict that began when Hamas attacked Israel, killing roughly 1,200 people and taking about 250 hostages. The Israeli military has since killed more than 67,000 Palestinians, including civilians and combatants, according to local health officials.

“Jared is a very smart guy,” Mr. Trump said on Thursday.

Trained in the bare-knuckle arts of New York City real estate, Mr. Kushner and Mr. Witkoff think of themselves as deal guys working for the ultimate deal guy. Their approach is simple: Get to a yes first, and hash out the details later. The two have spent a lot of time together over the past few weeks, crisscrossing Miami, then the country, and now the world, in pursuit of peace. The rebuilding of Gaza is also in their sights.

“The experience that Steve and I have as deal guys is that you have to understand people,” Mr. Kushner said. “You have to be able to kind of get the bottom line out of them, and then see who do you think is playing games, and how much room do you have to push things?”

He added: “A lot of the people who do this are history professors, because they have a lot of experience, or diplomats. It’s just different being deal guys — just a different sport.”

Diplomacy and Business

Mr. Kushner has received bipartisan praise for his role in the negotiations, but as an unpaid volunteer, he is not subject to the same laws and disclosure requirements of a government employee. And Mr. Kushner has extensive business dealings in the Middle East, enriching himself as he builds deep diplomatic relationships with leaders across the region. His critics have said that he is evading bureaucratic safeguards designed to prevent conflicts of interest.

As for any questions about Mr. Kushner’s moneymaking efforts in a region of the world where he is trying to negotiate peace, the White House has said there is no problem.

“I think it’s frankly despicable that you’re trying to suggest that it’s inappropriate for Jared Kushner, who is widely respected around the world and has great trust and relationships with these critical partners in these countries, to strike a 20-point, comprehensive, detailed peace plan that no other administration would ever be able to achieve,” Karoline Leavitt, the White House press secretary, said last week when asked about whether his involvement was appropriate.

She added: “We are very proud of that plan, and we hope Hamas will accept it, because it will lead to a more peaceful and prosperous Middle East.”

Mr. Kushner’s private equity firm, Affinity Partners, is almost entirely financed from overseas investors, and has taken money from government wealth funds in Saudi Arabia, Qatar and the United Arab Emirates.

“In order for the next stage of this agreement to be successful, Jared Kushner needs to stop approaching this issue as a real estate deal and start focusing on political and human rights,” Senator Chris Van Hollen, Democrat of Maryland, said in a statement.

If Mr. Kushner seems like a familiar face in the Trump White House, that is because he never really left. He was an unpaid senior adviser to Mr. Trump during the president’s first term, as was his wife, Ivanka Trump, the president’s eldest daughter. Both faced intense criticism from Democrats for not being moderating forces with Mr. Trump, particularly on his efforts to restrict immigration.

The onslaught of headlines about their unpopularity were so constant that Mr. Trump would sometimes joke that instead of marrying the football player Tom Brady, she’d married Mr. Kushner. “Jared hasn’t been so good for me,” Mr. Trump would remark at points.

Relations have since improved.

Mr. Kushner began focusing on matters from prison reform to normalizing relations between Israel and several Arab nations. He left the White House, and his private equity firm has made money from his relationships with some of those countries. His father, Charles Kushner, was given a presidential pardon for tax evasion and retaliating against a federal witness, among other violations. The elder Mr. Kushner, a campaign donor, is now the U.S. ambassador to France.

‘The Right Timing’

Mr. Kushner has stayed in his father-in-law’s orbit as a volunteer adviser, and he has long abandoned any pretense that he would be the one to mediate Mr. Trump’s impulses. Instead, he has leaned heavily into Mr. Trump’s approach to achieving peace in Gaza, a process in which the president has made decisions on gut instinct over careful deliberation.

Michael Herzog, the former Israeli ambassador to the United States, said the combination of Mr. Kushner’s connections in the Middle East and his status as a member of the Trump family made him particularly valuable during the negotiations.

“I believe Jared played a role in convincing President Trump, together with Steve Witkoff, to come out with this initiative,” he said of the 20-point peace plan. “It was the right timing. Everyone around here didn’t give it much chance, but it worked.”

Mr. Kushner’s efforts are even winning praise from Democrats. Thomas R. Nides, who served as the U.S. ambassador to Israel in the Biden administration, said he was thrilled the final hostages were set to be released.

“He was exceptionally important in Abraham Accords, knows how to manage Bibi and understands the Arab countries,” Mr. Nides said of Mr. Kushner. “Forget my politics, I’m more than happy to give him as much credit as he deserves.”

Until recently, Mr. Kushner had a much less visible role in the president’s orbit. He was frequently in touch with his father-in-law and some of the president’s top aides, and made occasional visits to the White House.

Roughly eight months ago, Mr. Kushner started working with Tony Blair, the former British prime minister, on plans for a postwar Gaza. In August, the two of them met with Mr. Trump and his top aides to discuss the future of the Gaza Strip. In the weeks since that Oval Office meeting, Mr. Kushner emerged as a key player in the negotiations, working closely with Israel and the Arab nations.

“This is way more than I kind of anticipated,” Mr. Kushner said in an interview. “I think there’s a chance that when I get home, my wife changes the locks on the house.”