She had been battling several age-related conditions, said her daughter, Ellyn Lindsay.
Marcus, who wrote for programs such as Peyton Place, One Life to Live and Knots Landing, in addition to sitcoms and variety shows, served several terms on the Writers Guild of America West board. She was an outspoken activist in the fight against the graylisting of older writers in youth-oriented Hollywood, a problem she said had gotten worse in the 1990s.
Among her early writing credits were episodes of the 1960s sitcoms Dennis the Menace and Please Don't Eat the Daisies.
Marcus was born Dorothy Ann Goldstone on Aug. 22, 1921, in Little Falls, N.Y. She earned a degree in sociology from Western College for Women in Oxford, Ohio, (now part of Miami University) in 1943.
Moving to New York, she lied about her age — saying she was older than she was — to get a job at Life magazine. But she bristled at how difficult it was for women in the profession to be taken seriously. In 1944 she was interviewing foreign policy authority John Foster Dulles when he got a phone call and told the caller, "I can't talk to you now. I'm being interviewed by a little girl from Life magazine."
She married writer Ellis Marcus in 1944 and took time out to start a family, moving to Los Angeles because he was getting work in television. She also wanted to work in the medium, and her play "A Woman's Place" got her noticed when it was staged in the early 1960s. Still, she could not get a TV assignment unless her husband wrote with her. That worked out well, since they enjoyed writing together. Soon she got work on her own, including on the nighttime serial Peyton Place, which brought her aboard in 1966.
Her favorite show was the soap opera spoof she co-created, Mary Hartman, Mary Hartman, about a small-town homemaker who had to deal with everything from "waxy yellow buildup" on her kitchen floor to mass murder. "She felt it was groundbreaking," her granddaughter Katie Lindsay said, "in the way it talked about female sexuality."
Making its debut in 1976, the comic soap became a cult hit, and Marcus shared that year's prime-time Emmy for outstanding program and individual achievement.
In a 2001 interview with the Academy of Television Arts and Sciences, Marcus noted that Mary Hartman was even studied in college courses. "We had no idea it was going to be taken that seriously," she said. "We were just trying to be funny, satirical, tweak a couple sacred cows. So it was quite a marvelous time."
Later in interview, Marcus discussed the censorship she faced during her tenure as head writer of Days of our Lives (1977-1979).
"I wanted to do an interracial story about a Vietnamese young woman who was the mother to a child one of the popular characters had fathered when he was in Vietnam," she explained. "He's about to get married to someone in this country and she turns up. That's the story I wanted to do."
NBC would not agree to do the story so Marcus' run at Days ended.
Marcus' other TV credits included All That Glitters, Search for Tomorrow, Love of Life, Love is a Many Splendored Thing, Falcon Crest, General Hospital and Flamingo Road.
Though Marcus fought ageism in the industry, she also bucked the trend, working on the nighttime soap Knots Landing into her 70s. Writer Lisa Seidman, who also worked on the show, said that no matter how busy they got, Marcus made time for her Writers Guild duties. "I would say to her, 'You can't leave, we're in the middle of a meeting!'" Seidman said. "And she would yell back, 'This is important! This is your future!'"
In addition to her daughter Ellyn Lindsay, who lives in Los Angeles, Marcus is survived by sons Steve Marcus of Santa Monica and John Marcus of Flagstaff, Ariz.; six grandchildren; and seven great-grandchildren. Ellis Marcus died in 1990.