One of the signature traits of Andy Jassy’s leadership of Amazon.com Inc.’s cloud-computing unit was a meeting tool called “the wheel.”

At weekly staff meetings, Mr. Jassy and other top executives would use the wheel to choose employees at random to give presentations on their business, according to current and former employees. Mr. Jassy and the executives would listen intently—perhaps interrupting to correct errors—and then pepper staff with detailed questions.

Starting Monday, Mr. Jassy will bring that ultra-detail-oriented management style to the monumental task of running all of Amazon. He fills some of the biggest shoes in business as he takes the chief executive role from the company’s centibillionaire founder, Jeff Bezos.

Mr. Jassy, 53 years old, is an Amazon lifer who learned the ropes at Mr. Bezos’s side. He shares some traits with his mentor, and in other ways is a different type of leader. People who’ve worked closely with Mr. Jassy over the years describe him as soft-spoken and approachable in a way that some say Mr. Bezos isn’t. An avid sports and music fan, Mr. Jassy likes to wear jeans to the office and can occasionally be spotted in restaurants around Amazon’s Seattle campus.

Though some former employees say Mr. Jassy is more even-keeled with employees than Mr. Bezos—who has been known to explode in meetings if an executive seems ill prepared—they also say he is at least as intense and competitive. And while Mr. Bezos for the past several years has largely governed with a hands-off approach, as The Wall Street Journal reported in February, Mr. Jassy is known for digging into the minutiae of his division, Amazon Web Services, sometimes to a degree that baffled his underlings, according to former employees. He did everything from pitching potential customers and guiding technical changes to choosing the music at company events and editing press releases. It was more difficult to reserve a spot on Mr. Jassy’s calendar than it was with any other top executive reporting to Mr. Bezos, according to former senior Amazon executives.

“He’s got just a phenomenal focus on details,” said James Hamilton, an Amazon vice president who has worked with Mr. Jassy for more than a dozen years. “That relentless focus on detail is truly unique.”

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Those traits enabled Mr. Jassy to help create and build Amazon Web Services, known as AWS, into a pioneer of the cloud-computing revolution and a major company in its own right. It has tens of thousands of employees, and with more than $45 billion in revenue last year, it would rank 69th on the Fortune 500 if it were independent, a few notches behind Best Buy Co. In earnings, AWS punches way above its weight: its operating profit was $13.5 billion last year, nearly 60% of Amazon’s total.

Its sprawling parent company, though, makes AWS look tiny in most regards. Amazon boasts nearly 1.3 million total employees, $386 billion in 2020 revenue—the second most of any U.S. company after Walmart Inc. —and businesses ranging from its core online retail platform to entertainment, advertising, logistics and healthcare.

Mr. Jassy also will now have to lead the company’s battle against lawmakers and regulators who are mounting their biggest antitrust effort in decades to target Amazon and other giants. The House Judiciary Committee last month approved a raft of proposed legislation targeting big tech companies, including a bill aimed substantially at Amazon that seeks to force a split between big companies’ online marketplaces and their in-house brands that sell on those platforms. That came shortly after President Biden appointed Lina Khan, a lawyer who gained fame largely over her criticisms of Amazon’s competitive tactics, to lead the Federal Trade Commission. The FTC is examining Amazon’s practices.

Mr. Jassy has had little involvement with the antitrust issues so far because AWS hasn’t figured prominently in those discussions. But Amazon’s critics have already set their sights on him. Hours after the plan for him to take over was announced in early February, Rep. Ken Buck of Colorado, the top Republican on the House Antitrust Subcommittee, fired off a message to him on Twitter. “I have some questions for Mr. Jassy,” he said.

Amazon declined to make Mr. Jassy available for comment. The company has said that it operates its business fairly and to the benefit of customers, and that the proposed legislation in the House would hurt both businesses that sell on its platform and hundreds of millions of consumers who shop there.

The ‘most Amazonian’

The son of a prominent corporate lawyer, Mr. Jassy grew up in the New York City suburb of Scarsdale, N.Y., where he played varsity soccer and tennis. One nickname listed in the 1986 Scarsdale High School yearbook was “Disco Jass,” and a quote below his picture read “Never do today what you can put off till tomorrow.” An Amazon spokesman said that the nickname was an inside joke.

He joined Amazon as a marketing manager in 1997, after completing an M.B.A. at Harvard University, where he’d also earned his undergraduate degree. It was only a few years after Mr. Bezos launched the company as an online bookseller from his garage. Four years younger than the founder, with his birthday just one day after Mr. Bezos’s, Mr. Jassy served in several roles, including as the founder’s technical adviser, a role that involved shadowing him full-time.

Mr. Jassy and a team began planning out what would become AWS between 2000 and 2003. Amazon had become skilled at overseeing the massive low-cost computer infrastructure needed to run its retail operations, and AWS became a way to sell that expertise externally. Amazon launched it in 2006 with Mr. Jassy at the helm. In 2009 he and his wife paid $3.15 million for a house less than 2 miles from Amazon headquarters in Seattle, according to property records. The couple now has two children ages 17 and 20.

Bill Carr, who worked at Amazon for more than 15 years and led its entertainment operations, said that Mr. Jassy stood out even in earlier years for being simpatico with Mr. Bezos. “We have this term people use, which is, are you being Amazonian?,” said Mr. Carr, who left in 2014. “Andy of all of Jeff’s reports was the most Amazonian.”

Andy Jassy, a sports fan, is a minority owner of Seattle’s soon-to-launch National Hockey League team, the Kraken. Here is a May 2020 rendering showing Climate Pledge Arena, where the Kraken will play, with Amazon's headquarters in the distance.

Andy Jassy, a sports fan, is a minority owner of Seattle’s soon-to-launch National Hockey League team, the Kraken. Here is a May 2020 rendering showing Climate Pledge Arena, where the Kraken will play, with Amazon's headquarters in the distance.

Photo: Climate Pledge Arena

Former colleagues say Mr. Jassy would spend enormous amounts of time on the narrowest of details if he thought it was important. He spent time choosing the artists for a music festival AWS started in 2019 to go along with its re:Invent event in Las Vegas. When AWS was launching a new data analytics product in 2012, said a former executive, he and a team took four weeks to decide on the name before settling on Redshift the day before it was to be unveiled at the cloud unit’s annual conference.

When an AWS data center in Virginia was hit by a major outage, Mr. Jassy personally got involved in figuring out the problem, recalled Mr. Hamilton. It turned out a technician had been checking a generator and the door accidentally bumped into a switch, shutting it off. Mr. Jassy dug into the incident and pressed the team to redesign the generators. “When the CEO is digging at that level, everyone at the company starts to dig at the same level,” said Mr. Hamilton.

Mr. Jassy’s time at Amazon has made him wealthy, though nowhere near as much as his boss, who tops global rich lists. Mr. Jassy’s compensation last year, including base salary of $175,000 plus stock grants, totaled $35.8 million—second to the new head of Amazon’s consumer business, Dave Clark, who was promoted in August. Mr. Jassy owned 88,538 shares in Amazon as of May, according to a company filing. That is worth about $300 million at current prices, and makes him the third largest individual shareholder after Mr. Bezos and his ex-wife, MacKenzie Scott. But Mr. Jassy’s stake amounted to less than 0.02% of the company. Mr. Bezos owned more than 10% of Amazon as of June, worth $175 billion.

Despite his wealth, Mr. Jassy is known by some for frugality. One of the former colleagues said he balked at the suggestion that he should use a private jet. “It makes him uncomfortable to waste money,” the former executive said. Another acquaintance said she was surprised to watch him get into his car after one meeting—it was an old beat up Jeep Cherokee. “I remember being so struck that this guy could drive any car he wanted,” said Sarah Smith, the former executive director of Rainier Scholars, an education nonprofit where Mr. Jassy has served on the board of trustees for over a decade.

His biggest private passions are music and sports. He’s a fan of the Foo Fighters and has kept a poster of a Dave Matthews Band album cover in his conference room. He has been known to host wing-eating contests at his man cave at home, where every year he also invites a team of his closest employees on the first Friday of March Madness for an all-day college-basketball watching party. The basement room features a glass-encased shadow box with memorabilia from major sporting events he has attended, said one of the people who has joined him. He is a minority owner of Seattle’s soon-to-launch National Hockey League team, the Kraken.

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‘Nice!’

Mr. Jassy can be more reserved than Mr. Bezos. He deliberately waits until everyone else in the room has spoken in a meeting before chiming in with questions, said current and former employees. Whereas many executives fear Mr. Bezos’s scathing assessments if he dislikes their presentations, Mr. Jassy is measured, even when expressing dissatisfaction, the people said. “I’ve never seen him speak in a way that’s anything but calm,” said Mr. Carr.

Amazon executives dread emails from Mr. Bezos with a question mark, but Mr. Jassy’s most common response to emails is “Nice!” He’s become so known for the response that some AWS employees made stickers with the CEO’s face on them with a conversation bubble from his mouth saying “Nice!”

One AWS executive recalls an all-hands meeting where Mr. Jassy said: “We’re not put on earth to work” with the executive stressing family balance. Mr. Bezos in the past has dismissed the concept of work-life balance, saying he preferred the concept of “work-life harmony.”

Mr. Jassy, here speaking during the 2017 Amazon Web Services Summit in San Francisco, would spend enormous amounts of time on the narrowest of details if he thought it was important, former colleagues said.

Mr. Jassy, here speaking during the 2017 Amazon Web Services Summit in San Francisco, would spend enormous amounts of time on the narrowest of details if he thought it was important, former colleagues said.

Photo: David Paul Morris/Bloomberg News

Still, Mr. Jassy is demanding. At the weekly “wheel” meetings, Mr. Jassy and his deputies would ask pointed, in-depth questions of whichever manager or team member was picked—they initially used a handmade spinner like those used in board games or game shows, but later adopted a computer algorithm when there were too many divisions to include. Some employees called it “the firing squad,” and some called it “the wheel of death.” An Amazon spokesman said these phrases are not common descriptions for the meeting.

When Mr. Jassy is displeased with an employee, he can leave very pointed feedback, though he’ll often follow up with a conciliatory email later, one of the people said. Mr. Jassy doesn’t yell in meetings, but can get very pointed in his critiques, especially if he feels someone is not taking a problem seriously enough. “The guy has a sense of urgency that I’ve never seen in my life,” said a former executive.

The wheel meeting is emblematic of Mr. Jassy’s view that you can’t rely on people’s good intentions—you need a forcing function to make them live up to company expectations, according to people familiar with Mr. Jassy. If the wheel wasn’t in place, some people wouldn’t come prepared. “He wanted everyone in the room to be able to dive into the minutiae of the service,” said one current AWS executive.

Big defections have angered him, according to people familiar with the matter. The company has initiated non compete lawsuits against former top employees he was very friendly with, some of the people said. “A perfect example of him using up a massive amount of effort and a lot of people to optimize something that isn’t very important,” said a former executive about one case where the company sued a former employee who left for Google. An Amazon spokesman said employee departures haven’t angered Mr. Jassy.

Mr. Jassy himself was once considered for the CEO position at Microsoft Corp. , according to people familiar with the matter. That job went to Satya Nadella in 2014.

Not long after, in 2016, Mr. Bezos named two of his top lieutenants CEOs of Amazon’s major businesses. Mr. Jassy was named CEO of AWS. Jeff Wilke was named CEO of Amazon’s expansive retail business. Earlier this year he left Amazon. AWS has nearly quadrupled its revenue in the years since.

Commander of the plane

Overseeing the parts of the business he didn’t run will be an adjustment for Mr. Jassy. AWS’s business focuses almost entirely on corporate and government customers, while Amazon’s retail and entertainment operations have hundreds of millions of individual users. But Mr. Jassy’s role on the S-team, the name given to the group of around 20 top executives under Mr. Bezos, has given him exposure to issues across the company. Mr. Hamilton, the AWS executive, said that despite Mr. Jassy’s love of detail, he’s good at delegating.

“AWS has grown to be an incredibly large, big business right now, and Andy has easily scaled to keep up with that,” he said.

Some Amazon investors say while they have high confidence in Mr. Jassy’s ability to steer Amazon successfully, the new CEO faces heightened scrutiny compared with running AWS.

“It’s like being a fighter pilot and having a lot of experience in the simulator, but regardless, when you go up there and get in your first dogfight, it’s different,” said Trip Miller, managing partner of Gullane Capital Partners LLC, which he said owns more than $37 million in Amazon stock. “You’re the commander of the plane.”

Mr. Bezos, who will remain executive chairman, has emphasized continuity in handing the CEO reins to Mr. Jassy. The founder, who chose July 5 as the handover date because it’s the 27th anniversary of the date Amazon was incorporated, has already in recent years delegated much of the day-to-day running of the company to Mr. Jassy and other senior lieutenants, the Journal has reported. This month, Mr. Bezos plans to travel to space with his brother as part of the first crew carried by his rocket company Blue Origin.

At Amazon’s annual shareholder meeting in May, Mr. Bezos said his successor “has the energy needed to keep alive in us what has made us special.” Mr. Jassy “has my full confidence,” Mr. Bezos said. “He has the highest of high standards, that I guarantee that Andy will never let the universe make us typical.”

—Sebastian Herrera contributed to this article.

Write to Aaron Tilley at aaron.tilley@wsj.com, Dana Mattioli at dana.mattioli@wsj.com and Kirsten Grind at kirsten.grind@wsj.com