WASHINGTON — Robert C. McFarlane, a former decorated Marine officer who rose in civilian life to be President Ronald Reagan’s national security adviser and then fell from grace in the Iran-contra scandal, died on Thursday in Lansing, Mich. He was 84.

Mr. McFarlane, who lived in Washington, was visiting family in Michigan at the time. A family friend, Bill Greener, said the death stemmed from an unspecified previous lung condition.

Mr. McFarlane pleaded guilty in 1988 to charges of withholding information from Congress in its investigation of the affair, in which the Reagan administration sold arms covertly to Iran beginning in 1985 in exchange for the freedom of Western hostages in Lebanon. Profits from the arms sales were then secretly funneled to the contra rebels in Nicaragua, who were trying to overthrow the country’s Marxist regime, known as the Sandinistas.

Both parts of the scheme were illegal; Congress had imposed an arms embargo against Iran and prohibited American aid to the contras.

Mr. McFarlane, Bud to his friends and associates, was one of many players in the operation, which was run out of the White House with the cooperation of the Central Intelligence Agency. But he distinguished himself in its aftermath by his full and unequivocal acceptance of blame for his actions. Everyone else involved had either defended the operation as just and wise or sought to deny responsibility.

The episode stained the Reagan administration and raised questions as to how much the president was aware of what was going on in his own White House.

And its fallout left Mr. McFarlane so ridden with guilt that he attempted suicide in his home in February 1987. While his wife, Jonda, a high school English teacher, was upstairs grading papers, he took an overdose of Valium and got into bed alongside her. When he couldn’t be roused in the morning, he was taken to a hospital and revived. He subsequently underwent many weeks of psychiatric therapy at the Bethesda Naval Hospital.

It was a stunning act in official Washington. Many considered it an unconcealed howl of pain by someone from whom they would have least expected it — one of the capital’s most self-contained of public and powerful men.

Killing himself, Mr. McFarlane believed at the time, was “the honorable thing to do,” he said in an interview for this obituary in January 2016 at his home in the Watergate complex in Washington.

“I so let down the country,” he said.

He earlier had tried to explain his actions by citing the ancient Japanese tradition of the honorable suicide. But he came to realize, he said in the interview, that those ways had no resonance in modern American culture and that most people could not understand such behavior.

Mr. McFarlane always asserted — and he was supported by evidence — that he had been involved mostly in the Iran part of the scandal, and that he had been ignorant of the more blatantly illegal portion, the sending of profits from the weapons sales to the Nicaraguan contras.

Mr. McFarlane had been a fervent advocate of repairing relations with Iran — so much so that after he left the White House he made a secret visit there in 1987, traveling incognito, at President Reagan’s request. There he met with various officials but found that the meetings were a waste of time, he said.

The results of the arms sales themselves were little better: A few hostages were released sporadically by Iran’s allies in Lebanon — fewer than had been promised — and, in any event, new hostages were subsequently seized.

The scheme began to unravel on Oct. 5, 1986, when a plane supplying arms to the contras was shot down in Nicaragua, exposing the mission and prompting an investigation by a joint congressional committee and televised hearings. Summoned to testify, Mr. McFarlane and his former deputy, Lt. Col. Oliver L. North — White House figures little known to the public until then — emerged into the glare of national publicity as key players in the affair.

Colonel North, still an officer on active duty at the time, was an enthusiastic player in the scheme. He held forth before a joint congressional investigating committee in full dress uniform (he had favored business suits in the White House), at times expressing defiance, at other times insisting that he had been motivated by patriotism.

Colonel North’s testimony made him a national hero to many conservatives, and he later parlayed that support into hosting a talk show, writing books and running, though unsuccessfully, for the United States Senate from Virginia as the Republican nominee. (He was later president of the National Rifle Association for less than a year.)

Mr. McFarlane, by contrast, did not garner any such public adulation or even much support. Job offers were withdrawn, he wrote, and he was asked to resign from a corporate board.

In his memoir, he recalled that at first he had liked Colonel North, his fellow Marine, and thought that they had much in common. That changed after he discovered, he said, that Colonel North had deceived him about many of his activities.

He wrote that in misjudging Colonel North he “did not see what was really there, the manipulative skill, the easy betrayal, the hubris and the fierce ambition for personal advancement.” He campaigned against him in the Virginia election.

Mr. McFarlane did, however, win approval from some of those who had investigated the Iran-contra affair.

One member of the investigating committee, Senator Daniel Patrick Moynihan, Democrat of New York, praised his testimony, saying that there was “no ‘cute,’ no evasion. ‘I’m here, I’ll tell you everything I know.’”

The independent prosecutor, Lawrence E. Walsh, who was frustrated by the stiff resistance of others who had been involved in the operation, acknowledged that he had been so moved by Mr. McFarlane’s forthrightness and contrition that he chose to charge him with only four misdemeanor counts.

Mr. McFarlane served a sentence of 200 hours of community service, in part by helping to establish an independent living program for the disabled in suburban Washington, and by setting up a computer program listing after-school recreational programs for area youths.

Before he left office, President George H.W. Bush pardoned Mr. McFarlane on Christmas Eve, 1992, along with others involved in the Iran-contra affair, including Defense Secretary Caspar W. Weinberger.

An unresolved question at the heart of the Iran-contra matter was the extent of President Reagan’s knowledge and support. The episode has been an important area of study for scholars pondering if Reagan — who after his retirement was acknowledged to have Alzheimer’s disease — had begun to lose his mental acuity in the White House. Mr. McFarlane, in interviews and in his memoirs, depicted the president as sometimes confused or vague about the details of what was happening with Iran and the contras. But he depicted Mr. Reagan as mostly in control.

Robert Carl McFarlane was born in Washington on July 12, 1937, the son of a Democratic congressman, William McFarlane, from the Texas Panhandle and a grandson of a Texas Ranger. Despite those roots, he was to have little Texas in him, growing up in the Washington area.

He graduated high in his class in 1959 from the Naval Academy in Annapolis; married his high school girlfriend, Jonda Riley; and joined the Marines. As a captain, he led one of the first combat operations in Vietnam. He described the operation as almost farcical.

His commanding general, he recalled in an interview, insisted that he take his troops ashore in a difficult waterborne landing, even though it would it have been easier to get to their destination by simply docking at a nearby pier. A shore landing was more suitable for Marines, the general told him. Mr. McFarlane said his heart sunk as he watched his command jeep plunge to the bottom of a hidden lagoon.

In the 2016 interview with The Times, Mr. McFarlane lamented that while he was the national security adviser, he did not press the basic lesson he thought he had learned in Vietnam: that the United States should not wage war without clear and strong support at home. He said the Reagan administration had been wrong to try to help the contras because there was little public support, as evidenced by the congressional ban on aid to them.

Mr. McFarlane was a surprise choice to succeed William P. Clark Jr. in October 1983 as Reagan’s second national security adviser, the person in the White House responsible for coordinating policy among the State and Defense Departments and other government agencies. He was generally considered a staff person, a notable contrast from some of his more well-known predecessors, who brimmed with abundant self-confidence and published scholarly works, like Henry A. Kissinger and Zbigniew Brzezinski.

He began his climb in the national security establishment while still a lieutenant colonel in the Marines, when he won a White House fellowship and worked for Mr. Kissinger and then Brent Scowcroft when they were national security advisers. He also held senior staff positions at the Senate Foreign Relations Committee and the State Department.

According to contemporary accounts, he played important roles in complicated and significant arms control negotiations with the Soviet Union and, especially, promoting and guiding President Reagan’s antimissile defense program known as Star Wars. The system was never put in place, but it was said to have forced Moscow to vastly accelerate military spending to the detriment of the Soviet Union, hastening its collapse.

After he left the government, Mr. McFarlane founded an international business consulting company specializing in energy issues.

His survivors include his wife; three children, Lauren, Melissa and Scott; two sisters; and eight grandchildren.

Jordan Allen contributed reporting.