Cooper's most notable rulings included the dismissal of espionage charges against Chinese-born spy suspect Katrina Leung and the sanctioning of Los Angeles city authorities in the wrongful death case of rapper Christopher Wallace, better known as Notorious B.I.G.
U.S. Supreme Court last year, Cooper defied the claims of President George W. Bush's administration that she was interfering with national security when she imposed conditions on Navy sonar testing to protect whales off the coast of Southern California. The high court upheld much of her position, although it overturned her on two key protections sought by the environmental groups that brought the action.
Cooper also presided over the two-decade copyright battle involving A.A. Milne's Winnie the Pooh character, ruling last year that the Walt Disney Co. had legal ownership of the lucrative cartoon trademark.
Cooper died at St. John's Health Center in Santa Monica after a recurring battle with lymphoma and two recent strokes. She had given notice last year of her plans to retire from the federal bench to take a position in private mediation, a difficult decision brought on by the need to provide costly institutional care for her husband of many years, Les Peckins, who has Alzheimer's disease. She also had diabetes, friends and colleagues said.
"It was a whole series of things, just one thing after another. All the drugs she was taking for the various problems just weakened her and her body just gave out," said longtime friend and neighbor Roxanna Galbreath.
U.S. District Judge Audrey Collins, chief judge for the Central District of California, in which Cooper served, said the entire legal community of Los Angeles was shaken by Cooper's unexpected death.
"We did know she was leaving her position in March, but this is a big difference," Collins said. "It's just an unbelievable loss."
Richard Kendall, a lawyer who represented environmental groups in Cooper's courtroom, including the Natural Resources Defense Council's challenge of the U.S. Navy over the sonar testing, described the late judge as a model of civility and fairness.
"She had this quiet and clear-headed courage of her convictions," Kendall said. "There was no self-advertising as someone defiant of authority. But when she dug into a case and decided the government needed to be called to account, she just didn't hesitate."
Cooper stirred anger and criticism among government attorneys when she dismissed the espionage charges against Leung in 2005, accusing the government of "willful and deliberate misconduct" in cutting a deal with her former lover and co-defendant, retired FBI agent James J. Smith, that put him off limits as a witness.
The judge also riled authorities when she declared a mistrial and imposed a $1.1-million sanction against the city of Los Angeles for failing to turn over documents that would have been useful to rapper Wallace's heirs in their effort to prove their claims that two rogue L.A. police officers were complicit in his slaying.
Although the Leung and Wallace cases are most often cited as Cooper's most important contributions as a judge, she told friends and the rare interviewer she met with that she was most proud of her decision that saved Rancho Los Amigos National Rehabilitation Center from closure and stopped the reduction of beds at County-USC Medical Center, said close friend and lawyer Miriam Vogel.
In a 2006 interview with the Daily Journal legal publication, Cooper defended what critics held were unduly tough rulings against government lawyers.
"I do believe that I hold the government to a higher standard," she said. "If we can't rely on the government to be honest, we are in great peril."
Even those who lost in her court tended to respect her, said Justice Nora Manella of the state Court of Appeal.
"She was smart as a whip, and funny. She just had a tremendous sense of humor and was unfailingly courteous in the courtroom," said Manella, who earlier served with Cooper in District Court.
Cooper was born Feb. 9, 1940, in Vancouver to a secretary and a Canadian Pacific Railway agent who moved the family to San Francisco when she was 12. Despite taking night classes at a Bay Area community college for five years, she hadn't earned a degree before entering the working world.
A young wife and mother at the time, Cooper hired on as a secretary to Bay Area attorney Frances Hancock, who recognized a razor-sharp intellect and innate sense of fairness that would serve her well on the bench, recalled her friend, mentor and first employer after law school, Arthur L. Alarcon, a senior judge on the U.S. 9th Circuit Court of Appeals.
"She was one of the outstanding lawyers and judges this country has produced. This is a tremendous loss to the bar and the judiciary," said Alarcon, who hired Cooper as his clerk after she graduated top in her class at what is now Whittier Law School in 1975.
"She was so good as a law clerk with me that after a while I could no longer tell which part of a decision I wrote and which part she wrote," Alarcon said of his years as a Superior Court judge. "She had become a part of my brain."
In addition to her husband, Cooper is survived by her daughter, Karen Albert; son, Joe Andrus; and stepdaughter, Angela Sample. She is also survived by a sister, Maureen Kelly Schulze, and three grandchildren.
Funeral arrangements were pending, according to the District Court, although Galbreath said the family planned services next week at the Kehillat Israel temple in Pacific Palisades.