Richard L. Thornburgh, a former crime-busting federal prosecutor who unflappably led Pennsylvania through the Three Mile Island nuclear crisis as the state’s two-term governor and served as U.S. attorney general from 1988 to 1991, died Dec. 31 at a retirement community in Oakmont, Pa. He was 88.

His son David Thornburgh confirmed his death but did not cite a specific cause.

In the summer of 1988, President Ronald Reagan needed to replace besieged Attorney General Edwin Meese III, who had resigned amid charges of ethics violations for mixing personal finances with government business and for allegedly helping cover up the White House’s role in the Iran-contra scandal. The administration sought a Republican with a law enforcement background and a track record of public integrity to take quick command of the Justice Department.

Mr. Thornburgh, who was tall, with a boyish, round face and horn-rimmed glasses, seemed an ideal candidate. Schooled in engineering and law, he was widely seen as methodical, effective and cool under extreme pressure.

As the U.S. attorney for western Pennsylvania from 1969 to 1975, he won convictions against organized-crime figures as well as police chiefs, city council members, mayors and other public officials who collectively took millions of dollars in bribes from mobsters.

For Mr. Thornburgh, the biggest professional challenge came not in a courtroom but rather in a trial-by-fire in crisis management when, as governor, he helped avert pandemonium during the Three Mile Island crisis in 1979, the most serious nuclear power plant accident in U.S. history.

He arrived in Washington amid high expectations to take control of a Justice Department reeling from Meese’s tenure.

Mr. Thornburgh served in the Reagan Cabinet for five months, then was asked to remain as attorney general in the new administration of George H.W. Bush even though some Republican leaders expressed doubts about his conservative bona fides. He was widely regarded as a GOP moderate, especially in contrast to Meese, a blunt and polarizing campaigner against abortion rights and affirmative action, and on other cultural flash points.

In the ensuing three years as U.S. attorney general, Mr. Thornburgh led the Justice Department during its investigation of the bombing of Pan American Flight 103 over Lockerbie, Scotland, as well as cases involving Colombian drug cartels and global money-laundering operations.

But the glare of national media scrutiny, harsh battles of political partisanship and legal turf wars took a toll on Mr. Thornburgh’s “Mr. Clean” reputation.

His department faced scrutiny for its slow pace — compared with those of state prosecutors — in pursuing prosecutions of Charles H. Keating Jr. and other fraudsters in the multibillion-dollar savings-and-loan crisis that had cost millions of Americans their life savings.

Mr. Thornburgh also was accused by congressional Democrats of protecting the White House in a tangled scandal dubbed “Iraqgate.” It appeared to involve members of the American and Italian governments, a multibillion-dollar bank fraud in the Atlanta branch of an Italian bank, and an arms buildup by Saddam Hussein’s Iraq amid the Iran-Iraq war of the 1980s.

One of the bankers went to prison for his role in making illicit loans. But the Justice Department, under Bill Clinton’s attorney general, Janet Reno, issued a report in 1995 absolving members of the Bush administration of misconduct.

One of Mr. Thornburgh’s policy triumphs as attorney general emerged from the Justice Department’s civil rights division. He served as the Bush administration’s point man in the passage of the 1990 Americans With Disabilities Act, which broadened the scope of civil rights for people with disabilities. He reassured lawmakers wary of the cost of new regulations on businesses, countering with the benefit to productivity and the economy from contributions by workers with disabilities.

The passage had been personally satisfying for Mr. Thornburgh, whose son Peter suffered from the effects of a traumatic brain injury in a car accident in 1960. The accident had also taken the life of Mr. Thornburgh’s first wife.

Richard Lewis Thornburgh was born in Rosslyn Farms, a prosperous suburb of Pittsburgh, on July 16, 1932. His family consisted almost entirely of engineers and Republican Party stalwarts.

He received a bachelor’s degree in engineering from Yale University in 1954 and graduated three years later from the University of Pittsburgh law school. He spent most of his early legal career with the law firm of Kirkpatrick & Lockhart in Pittsburgh.

He said the car accident that killed his wife, the former Virginia Hooton, and severely injured his son prompted soul-searching about his future.

He was remarried in 1963 to a former schoolteacher and three years later sought public office, running for the U.S. House of Representatives on a platform that included advocating for civil rights initiatives and de-escalating U.S. involvement in the Vietnam War. He lost the race.

In 1969, the newly elected Republican president, Richard M. Nixon, named Mr. Thornburgh the U.S. attorney for the Western District of Pennsylvania.

His diligence in prosecuting cases caught the attention of higher-ups in Washington and, in 1975, he was elevated to assistant attorney general in charge of the Justice Department’s criminal division. The next year, he helped create the public integrity section to investigate allegations of political corruption.

In 1978, he won the gubernatorial race against former Pittsburgh mayor Peter F. Flaherty. Nothing in the campaign could have prepared him for what unfolded eight weeks into his first term.

On the morning of March 28, 1979, while meeting with state lawmakers about budget issues, Mr. Thornburgh received a phone call that there had been an accident at the Three Mile Island nuclear power plant, located on a sandbar in the middle of the Susquehanna River about 10 miles downstream from the state capital, Harrisburg.

A chain of events involving mechanical failure, design flaws and human error led to the partial meltdown of the reactor core in Unit 2 at the nuclear power plant.

Mr. Thornburgh urged residents in the surrounding area to remain calm as he tried to get a grasp on what was happening at the plant. Using his prosecutorial questioning skills to cut through contradictory information during the early days of the crisis, he determined that the situation wasn’t as bad as some had feared but that government officials needed to remain vigilant.

After engineers regained control of Three Mile Island, Mr. Thornburgh led President Jimmy Carter and first lady Rosalynn Carter on a tour of the facility to help put a jittery public at ease.

Mr. Thornburgh spent many more years working on the Three Mile Island cleanup efforts, but he also focused his attention on the state’s declining industrial-based economy. He cut personal and business tax rates and balanced the state’s budgets for each of his eight years in office. He also helped forge partnerships to lure technology companies.

“He really understood the evolution of the old economy of coal, iron and steel to the new economy of finance, real estate and technology,” said G. Terry Madonna, a political scientist at Franklin & Marshall College in Lancaster, Pa. “After Three Mile Island, which he handled brilliantly, with calm and deliberate decision-making, his job approval soared.”

Prohibited by state law from running for a third term, Mr. Thornburgh was soon in Washington as the newly appointed U.S. attorney general.

In 1991, he left the Justice Department when Senate Republican leaders persuaded him to run in a special election for the U.S. Senate seat from Pennsylvania after the death of John Heinz (R-Pa.) in a plane crash. Painted as a Washington insider, he was defeated in a stunning upset by Harris Wofford, a former president of Bryn Mawr College, who rode anti-Bush sentiments to victory.

Survivors include his wife, the former Ginny Walton Judson of Oakmont; three sons from his first marriage, John Thornburgh of Wexford, Pa., David Thornburgh of Philadelphia and Peter Thornburgh of Pittsburgh; a son from his second marriage, William Thornburgh of Pittsburgh; six grandchildren; and five great-grandchildren.

Mr. Thornburgh, who became counsel to the law firm K & L Gates in Washington, continued to give speeches about the value of holding elective office.

“Democracy is not a spectator sport,” he said in a 2009 address at the University of Pennsylvania. “And politics is an honorable calling. All of us must exercise the opportunity to contribute to improving and sustaining higher levels of performance in public life. This involves much more than simply voting or even being part of a focus group or responding to poll questions. And it is just as important in contests for the local school board as in those for higher office.”