Brett Sciaroni, a Reagan administration lawyer who was embroiled in the Iran-contra scandal of the 1980s and later became a power broker for the global business community in Cambodia under Hun Sen’s authoritarian regime, died March 12 at his home in Phnom Penh. He was 69.

His brother, Brian Sciaroni, said the cause was probably a heart attack, although an autopsy had yet to determine an official cause.

Mr. Sciaroni had just left his position as counsel to the President’s Intelligence Oversight Board, a panel that advises on the legality of U.S. intelligence operations, when he burst into the news in 1987 as Congress began hearings on Iran-contra.

Orchestrated by Marine Lt. Col. Oliver L. North and other members of President Ronald Reagan’s National Security Council, the illegal scheme involved the sale of arms to Iran to win the release of U.S. hostages held by Iranian-backed militants in Lebanon. The profits were then used to support right-wing Nicaraguan rebels known as the contras.

The plan was a blatant violation of congressional amendments prohibiting U.S. funding to topple the leftist regime in Nicaragua. The laws were passed between 1982 and 1984, the last of them after news reports emerged about CIA mining of Nicaraguan harbors and other unsavory tactics.

But in 1985, Mr. Sciaroni drafted a legal opinion that seemed to exempt NSC staffers from complying with the ban on aid. According to news accounts, even national security adviser Robert McFarlane — who publicly volunteered his role in the illicit Iran-contra affair — found the legal conclusion dubious.

Rep. Edward P. Boland (D-Mass.), who had sponsored the amendments to end U.S. assistance to the contras and served on a House select committee examining the Iran-contra affair, told the New York Times that Mr. Sciaroni’s testimony before the body in 1987 was “very useful” because it showed how the White House had tried to circumvent the law.

Mr. Sciaroni saw his reputation tarnished, in part because of revelations that he had failed four times to pass the bar exam and had never before held a job in a law-related field. His main qualifications for the oversight board had seemingly been his fealty to the Republican Party and his connections in conservative circles.

After working as a fellow at the American Conservative Union and lobbying in Washington on behalf of the rightist government of El Salvador, Mr. Sciaroni saw a chance at a fresh start in Cambodia. A congressional ally had advised him about a job opening as a legal adviser for one of the parties running in the 1993 election — the country’s first U.N.-backed vote following the reign of the Khmer Rouge and years of civil war.

“When I arrived out here, it was a single male-only posting,” he told Forbes, which in 2016 profiled him in a story headlined “The Most Powerful American In Cambodia.” “It was the Wild West in those days. Every cop on the street corner had an AK-47.”

His client was Prime Minister Hun Sen, who was running to retain the post. Although Hun Sen suffered a decisive loss in the election, Mr. Sciaroni helped his Cambodian People’s Party (CPP) strike a power-sharing agreement with the winning royalist party. Hun Sen then served as “second prime minister” for four years before seizing exclusive power in a 1997 coup.

As one of the first American lawyers in the country, Mr. Sciaroni became a sought-after counselor for the CPP and within a few years was made an official adviser to the government, a position that came with the ceremonial rank of minister. His primary role seemed to be to attract foreign capital and serve as a go-between for business executives seeking audiences with top Cambodian government officials.

In 1994, Mr. Sciaroni helped secure the country’s first major foreign investment, a $60 million brewery by Tiger Beer, a Singaporean brand produced by an arm of Heineken. The next year, he joined the Thai law firm Tilleke & Gibbins and established a satellite office in Phnom Penh. He also helped found the American Chamber of Commerce in Cambodia.

In the early 2000s, he negotiated a lower tax rate for American packaging company Crown Cork, which wanted to open a can manufacturing plant in the country but balked at the high aluminum import tax rate.

“In rapid succession we met various senior officials,” Mr. Sciaroni once told a Cambodian investment conference, according to Salon. “[One of them] said, ‘What would you like it [the rate] to be?’ and the company said, ‘How about zero percent?’ And zero percent it was and is today.”

Through Sciaroni & Associates, the law firm he started in 2004, and his earlier work, Mr. Sciaroni was credited with helping bring millions of dollars of foreign investment into Cambodia from Chevron, Ford, the Japanese conglomerate Mitsui and the Australia mining giant BHP Billiton, among other companies.

Mr. Sciaroni also became a trusted contact for the U.S. Embassy. William E. Todd, who served as U.S. ambassador to Cambodia from 2012 to 2015 and is now deputy undersecretary of state for management, said Mr. Sciaroni was a “generous and kind man [who] worked hard to improve the lives of the Cambodian people.”

Even as human rights groups condemned Hun Sen as a ruthless tyrant, Mr. Sciaroni gravitated toward superlatives and gave him the benefit of all doubt.

“I view him as the key transitional figure for Cambodia because he’s trying to bridge the gap between the old way of doing things and entry into the modern world,” he told Forbes. “You have political factions, economic factions, regional factions. And his job is to keep everybody on the same page more or less, and that’s not an easy job for a country like this.”

Bretton George Sciaroni, the middle of three sons, was born in Hollywood on Sept. 2, 1951, and grew up in Fresno, Calif. His father was a doctor, his mother a homemaker.

He graduated in 1973 from what is now Claremont McKenna College in California and received a master’s degree in international relations from Georgetown University in 1977. He completed a law degree the next year at the University of California at Los Angeles.

He worked at conservative policy groups including the Hoover Institution and the American Enterprise Institute and as a Reagan political appointee at the Commerce Department before joining the Intelligence Oversight Board.

Survivors include his wife of 15 years, Bui Thi Hoa My, and their daughter, Patricia, both of Ho Chi Minh City; and two brothers.

Mr. Sciaroni was long trailed by accusations that he was chief architect of a “white paper” that sought to justify Hun Sen’s bloody coup of 1997, during which at least 41 supporters of the royalist opposition were killed. He declined to comment on the paper but said his main role had been handling Hun Sen’s public relations and trying to make inroads for the regime with the GOP in Washington.

“My conservative brethren don’t understand my involvement with the CPP,” he told the New York Times. “I would argue the CPP is more pro-free enterprise. But ideology has nothing to do with it. It’s all power.”