"After a long and brave battle with cancer, our beloved Farrah has passed away," O'Neal said. "Although this is an extremely difficult time for her family and friends, we take comfort in the beautiful times that we shared with Farrah over the years and the knowledge that her life brought joy to so many people around the world."
Other "Charlie's Angels" stars paid tribute to her.
"Farrah had courage, she had strength, and she had faith. And now she has peace as she rests with the real angels," Jaclyn Smith said.
Said Cheryl Ladd: "She was incredibly brave, and God will be welcoming her with open arms."
Fawcett burst on the scene in 1976 as one-third of the crime-fighting trio in TV's "Charlie's Angels." A poster of her in a clingy swimsuit sold in the millions.
Her full, layered hairstyle became all the rage, with girls and women across America adopting the look.
She left the show after one season but had a flop on the big screen with "Somebody Killed Her Husband." She turned to more serious roles in the 1980s and 1990s, winning praise playing an abused wife in "The Burning Bed."
She had been diagnosed with cancer in 2006. As she underwent treatment, she enlisted the help of O'Neal, who was the father of her now 24-year-old son, Redmond.
This month, O'Neal said he asked Fawcett to marry him and she agreed. They would wed "as soon as she can say yes," he said.
Her struggle with painful treatments and dispiriting setbacks was recorded in the television documentary "Farrah's Story." Fawcett sought cures in Germany as well as the United States, battling the disease with iron determination even as her body weakened.
"Her big message to people is don't give up, no matter what they say to you, keep fighting," her friend Stewart said. NBC estimated the May 15, 2009, broadcast drew nearly 9 million viewers.
In the documentary, Fawcett was seen shaving off most of her trademark locks before chemotherapy could claim them. Toward the end, she's seen huddled in bed, barely responding to a visit from her son.
Fawcett, Kate Jackson and Smith made up the original "Angels," the sexy, police-trained trio of martial arts experts who took their assignments from a rich, mysterious boss named Charlie (John Forsythe, who was never seen on camera but whose distinctive voice was heard on speaker phone.)
The program debuted in September 1976, the height of what some critics derisively referred to as television's "jiggle show" era, and it gave each of the actresses ample opportunity to show off their figures as they disguised themselves in bathing suits and as hookers and strippers to solve crimes.
Backed by a clever publicity campaign, Fawcett - then billed as Farrah Fawcett-Majors because of her marriage to "The Six Million Dollar Man" star Lee Majors - quickly became the most popular Angel of all.
Her face helped sell T-shirts, lunch boxes, shampoo, wigs and even a novelty plumbing device called Farrah's faucet. Her flowing blond hair, pearly white smile and trim, shapely body made her a favorite with male viewers in particular.
A poster of her in a dampened red swimsuit sold millions of copies and became a ubiquitous wall decoration in teenagers' rooms.
Thus the public and the show's producer, Spelling-Goldberg, were shocked when she announced after the series' first season that she was leaving television's No. 5-rated series to star in feature films. (Ladd became the new "Angel" on the series.)
But the movies turned out to be a platform where Fawcett was never able to duplicate her TV success. Her first star vehicle, the comedy-mystery "Somebody Killed Her Husband," flopped and Hollywood cynics cracked that it should have been titled "Somebody Killed Her Career."
The actress had also been in line to star in "Foul Play" for Columbia Pictures. But the studio opted for Goldie Hawn instead. "Spelling-Goldberg warned all the studios that that they would be sued for damages if they employed me," Fawcett told The Associated Press in 1979. "The studios wouldn't touch me."
She finally reached an agreement to appear in three episodes of "Charlie's Angels" a season, an experience she called "painful."
She returned to making movies, including the futuristic thriller "Logan's Run," the comedy-thriller "Sunburn" and the strange sci-fi tale "Saturn 3," but none clicked with the public.
Fawcett fared better with television movies such as "Murder in Texas," "Poor Little Rich Girl" and especially as an abused wife in 1984's "The Burning Bed." The last earned her an Emmy nomination and the long-denied admission from critics that she really could act.
As further proof of her acting credentials, Fawcett appeared off-Broadway in "Extremities" as a woman who is raped in her own home. She repeated the role in the 1986 film version.
Not content to continue playing victims, she switched type. She played a murderous mother in the 1989 true-crime story "Small Sacrifices" and a tough lawyer on the trail of a thief in 1992's "Criminal Behavior."
She also starred in biographies of Nazi-hunter Beate Klarsfeld and photographer Margaret Bourke-White.
"I felt that I was doing a disservice to ourselves by portraying only women as victims," she commented in a 1992 interview.
In 1995, at age 50, Fawcett posed partly nude for Playboy magazine. The following year, she starred in a Playboy video, "All of Me," in which she was equally unclothed while she sculpted and painted.
She told an interviewer she considered the experience "a renaissance," adding, "I no longer feel ... restrictions emotionally, artistically, creatively or in my everyday life. I don't feel those borders anymore."
Fawcett's most unfortunate career moment may have been a 1997 appearance on David Letterman's show, when her disjointed, rambling answers led many to speculate that she was on drugs. She denied that, blaming her strange behavior on questionable advice from her mother to be playful and have a good time.
In September 2006, Fawcett, who at 59 still maintained a strict regimen of tennis and paddleball, began to feel strangely exhausted. She underwent two weeks of tests and was told the devastating news: She had anal cancer.
O'Neal, with whom she had a 17-year relationship, again became her constant companion, escorting her to the hospital for chemotherapy.
"She's so strong," the actor told a reporter. "I love her. I love her all over again."
She struggled to maintain her privacy, but a UCLA Medical Center employee pleaded guilty in late 2008 to violating federal medical privacy law for commercial purposes for selling records of Fawcett and other celebrities to the National Enquirer.
"It's much easier to go through something and deal with it without being under a microscope," she told the Los Angeles Times in an interview in which she also revealed that she helped set up a sting that led to the hospital worker's arrest.
Her decision to tell her own story through the NBC documentary was meant as an inspiration to others, friends said. The segments showing her cancer treatment, including a trip to Germany for procedures there, were originally shot for a personal, family record, they said. And although weak, she continued to show flashes of grit and good humor in the documentary.
"I do not want to die of this disease. So I say to God, `It is seriously time for a miracle,"' she said at one point.
Born Feb. 2, 1947, in Corpus Christi, Texas, she was named Mary Farrah Leni Fawcett by her mother, who said she added the Farrah because it sounded good with Fawcett. She was less than a month old when she underwent surgery to remove a digestive tract tumor with which she was born.
After attending Roman Catholic grade school and W.B. Ray High School, Fawcett enrolled at the University of Texas at Austin. Fellow students voted her one of the 10 most beautiful people on the campus and her photos were eventually spotted by movie publicist David Mirisch, who suggested she pursue a film career. After overcoming her parents' objections, she agreed.
Soon she was appearing in such TV shows as "That Girl," "The Flying Nun," "I Dream of Jeannie" and "The Partridge Family."
Majors became both her boyfriend and her adviser on career matters, and they married in 1973. She dropped his last name from hers after they divorced in 1982.
By then she had already begun her long relationship with O'Neal. Both Redmond and Ryan O'Neal have grappled with drug and legal problems in recent years.
Associated Press TV Writer Lynn Elber contributed to this report.