When the call came asking him to take on one last big assignment, the then-74-year-old Walsh said yes, embarking on a six-year journey digging into the crimes of Iran-Contra.
His detractors and there were many said his seemingly unending investigation was a clear case of prosecutorial abuse.
Iran-Contra paled in comparison to the Watergate scandal that brought down President Richard M. Nixon. But both were Washington spectacles: a collision of the executive and legislative branches of government, televised congressional hearings, a presidency in peril, an alleged criminal cover-up and criminal prosecutions that were, in Iran-Contra, all overseen by Walsh.
"I found myself at the center of a constitutional maelstrom," Walsh wrote in his 1997 book, "Firewall: The Iran-Contra Conspiracy and Cover-up."
Walsh "was really guided by a sense of intense personal responsibility for trying to do the right thing," said one of his former prosecutors, Michael Bromwich.
"For all the baseless charges that he was political and all through the many frustrations, he took his public service incredibly seriously and at great personal cost. His wife was quite ill. He had this killing schedule and he gutted it out. That's a level of sacrifice we don't have a right to expect from people called to public service. But it's the level of effort and sacrifice he was willing to give. He was truly a patriot and he was truly offended by corruption he saw at high levels in the U.S. government."
Iran-Contra involved two covert operations directed from the Reagan White House. In both, Congress was kept in the dark. The first operation was the secret supplying of weapons to rebels in Central America who were seeking to overthrow the leftist government of Nicaragua. At the time, Congress had prohibited U.S. military aid to the Contra rebels.
The second operation was the secret sale of arms to Iran in an effort to free U.S. hostages held in Lebanon. Iran was thought to possess some influence over the hostage-takers. The White House linked the two operations by secretly diverting millions of dollars from the Iran arms sales into buying guns for the rebels in Central America.
The disclosure of the diversion triggered a political firestorm, leaving the Reagan administration with little choice but to call for a criminal investigation by a wholly independent prosecutor. As a court-appointed independent counsel selected by three federal appeals judges, Walsh was given a broad mandate to pursue wrongdoing.
Walsh's investigation was a two-phase undertaking.
First, he prosecuted key figures, including former Marine Lt. Col. Oliver North, national security adviser John Poindexter and private-sector operatives who helped move missiles to Iran and guns to the Contras.
Eleven people pleaded guilty or were convicted by juries in Iran-Contra, but the two biggest courtroom victories for Walsh's prosecutors convictions of Poindexter and North were overturned on appeal.
Congress granted both men limited immunity from prosecution in exchange for their testimony in nationally televised hearings. The congressional immunity deals spelled the death knell for both criminal cases.
Critics urged Walsh to end the probe.
"To me, the real mark of Judge Walsh in terms of his courage and his commitment to public service was that he chose after that first phase not to simply say, 'We're done, I'm leaving,' but to complete the task that had been assigned to him," said Craig Gillen, a prosecutor on Walsh's team. "All of that information obtained in that second phase would simply not have been known to the country and history if it had not been his willingness to persevere."
It was in the second phase that Walsh's prosecutors discovered 1,700 pages of notes by Defense Secretary Caspar Weinberger that were, as Walsh put it, the equivalent of a reel of motion-picture frames. Many of the pages documented meetings inside the White House on Iran-Contra.
Other previously unrevealed notes some by aides to Secretary of State George Shultz, others by White House chief of staff Don Regan showed that Reagan and his Cabinet had known far more than they had admitted.
"The underlying facts of Iran-Contra are that, regardless of criminality, President Reagan, the secretary of state, the secretary of defense, and the director of central intelligence and their necessary assistants committed themselves, however reluctantly, to two programs contrary to congressional policy and contrary to national policy," Walsh said in a final report.
"They skirted the law, some of them broke the law, and almost all of them tried to cover up the president's willful activities," Walsh's report concluded.
Just as Walsh's office was preparing to present evidence of a high-level cover-up in court, President George H.W. Bush blocked the prosecution of Weinberger by pardoning him and five other Iran-Contra figures.
The pardons infuriated Walsh, who said the Iran-contra cover-up had continued for more than six years and "has now been completed."
In the end, the Iran-Contra probe cost $47 million and resulted in just one person being sent to prison a retired CIA officer who helped arrange weapons shipments to the Contras.
Reagan called Walsh's final report "a vehicle for baseless accusations that he could never have proven in court." Bush said that "at the heart of this investigation was a political dispute between the executive and legislative branches over foreign policy. We must be careful not to criminalize constitutional disputes of this kind."
After the Iran-Contra affair wound down, another court-appointed independent counsel Ken Starr came under fire for his five-year pursuit of President Bill Clinton in the Whitewater and Monica Lewinsky scandals.
The probes by Walsh and Starr left an indelible impression on politicians in Washington.
Walsh's long-running investigation put Republicans on the defensive politically and Starr's probe did the same to Democrats.
As a result, Congress refused to reauthorize the system of court-appointed independent counsels where prosecutors operate outside the purview of the Justice Department.
Lawrence Edward Walsh was born in 1912 in the Canadian province of Nova Scotia. His family moved to New York City when he was a small child. He graduated from Columbia University in 1932 and earned his law degree there in 1935.
As a young prosecutor in the office of a legendary racket-busting New York district attorney, Thomas Dewey, Walsh and the rest of Dewey's team of aggressive lawyers took on organized crime in the city. When Dewey was elected governor, he appointed Walsh his assistant legal counsel.
Later, Dewey named Walsh general counsel of the waterfront commission, where Walsh helped clean up crime on the New York and New Jersey docks, a battle that first brought Walsh into the public eye.
In 1954, President Dwight Eisenhower appointed Walsh to a federal judgeship.
Three years later, as the civil rights movement gained momentum, Walsh was appointed the Justice Department's deputy attorney general and given the responsibility of overseeing the continued desegregation of public schools after military forces had been withdrawn from Little Rock, Ark.
Walsh was a principal draftsman of the Civil Rights Act of 1960, which provided for the appointment of referees to help African Americans register to vote.
After John F. Kennedy defeated Nixon for president that year, Walsh returned to private practice. For the next 20 years, Walsh mostly specialized in litigation for corporations such as AT&T and General Motors.
By 1981, Walsh had moved to Oklahoma City, his wife's hometown, where he continued to practice law.
Walsh is survived by five children Barbara Marie Walsh, Janet Maxine Walsh, Sara Porter Walsh, Dale Edward Walsh and Elizabeth Porter Walsh as well as four grandchildren and two great-grandchildren.
His wife, Mary Porter Walsh, died in 2012.
The family said there would be a funeral service in Oklahoma City early next week and a memorial service in New York City at a later date.