RUTHFULLY YOURS

Prince Mohammed had worked doggedly for a year, outmaneuvering rivals and easing the path for his septuagenarian father Salman to assume the Saudi throne. After Salman took the crown in early 2015, he delegated extraordinary powers to Mohammed, who consolidated control of the military and security services and began upending the sleepy kingdom’s oil-dependent economy.

By that July, Mohammed wanted a break. His privacy-obsessed entourage booked the entire Velaa resort for a month, at a cost of $50 million, according to people familiar with the trip. Staff were banned from bringing cellphones with cameras. The American rapper Pitbull and the South Korean pop star Psy performed. The Maldives party was described by several people in attendance, including some involved in its planning.

During this time of excess, the young prince also bought the Serene—a 439-foot yacht rented by Bill Gates the year before—for 429 million euros, as well as a château near Versailles with fountains and a moat for more than $300 million.

Suddenly, the party was over. Word of Prince Mohammed’s visit leaked in a Maldivian publication. Media in Iran, Saudi Arabia’s enemy, picked it up and started blasting out the news. Less than a week after the trip started, Prince Mohammed and his delegation were gone.

The little-known party was a brief stop on Prince Mohammed’s wild ride. Over the next few years, he would curb Saudi Arabia’s notorious religious police, expand the rights of Saudi women, plunge into a war in Yemen, and lock up billionaires and relatives in Riyadh’s Ritz-Carlton Hotel on corruption allegations. His security forces would also assassinate the journalist Jamal Khashoggi in Istanbul. (Prince Mohammed has said he didn’t order the killing.) Now, with the kingdom’s 84-year-old King Salman suffering health problems that have him in and out of the hospital—recently for a gallbladder operation—understanding the young prince’s rise is increasingly important for assessing the direction Saudi Arabia may take if Prince Mohammed becomes King Mohammed.

The Saudi heir apparent’s ostentatious spending and focus on wealth grew from a family secret. For much of Prince Mohammed’s life, his branch of the family was nowhere near as rich as others. As a senior prince, his father Salman received a huge monthly stipend but spent it running his palaces, paying staff and doling out largess.

It hit 15-year-old Mohammed like a brick when a cousin told him that Salman hadn’t amassed what Saudi royals deem a serious fortune. Even worse, Salman was dangerously indebted. Family friends were shocked in the first decade of the 2000s when word spread through Paris that Salman’s contractors and employees hadn’t been paid for six months. Rather than relying on the good graces of whoever was king, Prince Mohammed decided to become the family businessman.

This account is based on interviews with dozens of people across three continents who knew Prince Mohammed through business, familial or personal ties. Prince Mohammed declined to comment on his business interests or rise to power. When a CBS News reporter asked him about his spending in a TV interview two years ago, the prince said, “As far as my private expenses, I’m a rich person and not a poor person.”

By the time he was 16, Mohammed was able to pull together about $100,000 after selling gold and luxury watches he’d been given. It became the starting capital for a stock-trading foray.

It was soon lost. But first, Prince Mohammed’s portfolio briefly gained value, giving him a thrill he would keep chasing. He decided to go abroad after university and get into banking, telecommunications or real estate. He wasn’t expecting real political power in Riyadh; as a younger son of a prince with slim chances of becoming king, Mohammed had little hope of getting near the throne.

Some evenings, Prince Mohammed took friends into the Saudi desert, where staff made up tents and a campfire. He talked of his plans to become a billionaire like Steve Jobs and Bill Gates. He also spoke with mounting frustration about Saudi youth. “We are the ones who can decide the future of our generation,” he said one night, an attendee recalls. “If we don’t step up, who else will?”

Prince Mohammed focused on building a fortune, creating companies and acquiring stakes in others. In 2008, through intermediaries, he persuaded Verizon to bring fiber-optic infrastructure to Saudi Arabia. The deal saw Verizon take a minority stake in a joint venture whose biggest partner was one of Prince Mohammed’s many firms.

Verizon’s legal department was then headed by William Barr, now the U.S. attorney general. The deal helped Prince Mohammed’s stature at home—“My son made millions for the family,” Salman boasted to one visitor after the deal closed—but didn’t go anywhere. Prince Mohammed’s organization didn’t have the experience to deliver the project; Verizon took a write-down and left.

Prince Mohammed’s stock trading also hit turbulence. In 2013, regulators detected suspicious trading patterns linked to accounts belonging to him and other princes. Saudi Arabia’s chief stock regulator at the time, Mohammed al Shaikh, investigated and determined that a trader acting on Prince Mohammed’s behalf was responsible for the suspicious trading. The incident incensed King Abdullah, and Prince Mohammed was pushed out of government affairs. (A spokesman for Prince Mohammed declined to comment on the episode.)

But it would be temporary. Even during his moneymaking days, Prince Mohammed was carefully studying how to gain—or, if necessary, regain—status in the Royal Court. He knew how to make himself useful to powerful old men like King Abdullah, performing tasks too distasteful for other princes, like evicting the widow of a former king from a palace she refused to vacate.

When Salman took the throne, Prince Mohammed’s ideas were suddenly the highest priority. The day after Abdullah’s funeral, Mohammed took charge of the Royal Court. At 4 a.m., he summoned officials and businessmen to meet later that day. Prince Mohammed asked them if remaking the Saudi government by getting rid of Abdullah’s governance committees was risky. Some counseled moving slowly to monitor for unforeseen impacts.

“Nonsense,” Prince Mohammed replied, according to a person who attended the meeting. “If it’s the right thing to do, we do it today.”

Within a week, Prince Mohammed was put in charge of the kingdom’s economy and military. Such haste would become a trademark. He surrounded himself with new advisers, some with little government background, and encouraged them to argue with him into the night on policy ideas. He elevated Mohammed al Shaikh—the man who investigated him in 2013 for insider trading—as a senior economic adviser.

Soon after, Prince Mohammed would take control of the state oil producer Saudi Aramco, the world’s most profitable company. With its money at his disposal, he would set to work turning the kingdom’s moribund sovereign-wealth fund into the most influential investor in Silicon Valley. The teenager obsessed with building his own fortune and emulating Steve Jobs now controlled more money than he knew what to do with. Building wealth and power became Prince Mohammed’s obsession.

—The authors are Wall Street Journal reporters. This essay is adapted from their book “Blood and Oil: Mohammed bin Salman’s Ruthless Quest for Global Power,” to be published Sept. 1 by Hachette Books.

Write to Bradley Hope at bradley.hope@wsj.com and Justin Scheck at justin.scheck@wsj.com