Johnson, a Palm Springs resident, died of esophageal cancer Monday at Cedars-Sinai Medical Center in Los Angeles, said her daughter, Heidi Johnson.
The sister of the late comedy writer Jerry Belson, Johnson began her nearly 40-year writing career in the 1970s when she wrote episodes of "The Mary Tyler Moore Show," "Laverne & Shirley" and other TV series.
She first teamed with Brooks to write (with Harry Shearer) "Real Life," a 1979 comedy that marked Brooks' feature film directorial debut.
Over the next 20 years, Brooks and Johnson co-wrote four other films directed by and starring Brooks: "Modern Romance" (1981), "Lost in America" (1985), "Mother" (1996) and "The Muse" (1999).
They also co-wrote (with Andrew Bergman) "The Scout," a Michael Ritchie-directed 1994 comedy starring Brooks and Brendan Fraser.
"Monica Johnson was an extraordinary person," Brooks said in a statement. "Funny, smart, and so much fun to work with. The world has lost a great sense of humor."
After "Mother," starring Debbie Reynolds in the title role, won Brooks and Johnson a New York Film Critics Circle Award for best screenplay, a Daily Variety story noted that people were saying: Monica who?
"They thought that I didn't exist. I was really Albert Brooks in drag," Johnson told the trade paper.
Because Brooks directed and starred in the films they wrote, she said, "he does overshadow everything. In the meantime, I would be this mystery woman. All the press would talk about is Albert."
Of her initial meeting with Brooks in the 1970s, she said, "it was just instantly like two peas in a pod. He felt the same way about humor based in reality as I did."
Born in Colorado on Feb. 21, 1946, Johnson was raised in El Centro, Calif.
A short bio that she wrote a couple of weeks ago for her website said she went to medical and dental assistants school "with a solid determination to marry a dentist. (She would have gone for an MD but had no self-esteem.)
"Then she got a lucky break: nepotism. Her brother introduced her to the world of comedy, and she hasn't looked back, except occasionally when she catches her coat in the door."
Her brother Jerry, a three-time Emmy Award-winning writer who died of prostate cancer in 2006, was a longtime writing partner of Garry Marshall. The pair developed and were executive producers of TV's "The Odd Couple."
Johnson was a single mother working as a clerk at a social services office in Los Angeles when she began making extra money typing "Odd Couple" scripts for her brother.
As Johnson typed, "she'd give the scripts a little punch-up, and he realized how funny his sister was," said Heidi Johnson. "Jerry put her together with writer Marilyn Suzanne Miller to write a spec script for 'Mary Tyler Moore.'"
Johnson went on to write for the sitcoms "The Paul Lynde Show," "Paul Sand in Friends and Lovers" and "Laverne & Shirley," for which she also was a producer.
Penny Marshall, who played Laverne in the hit series, recalled that Johnson and Jerry Belson had "totally different" senses of humor.
"He was much more bitter in his humor; he was harsher," Marshall, a longtime close friend of Johnson's, told The Times on Wednesday. "And she was eccentric."
Johnson's eccentricity extended to her work attire.
"She wore a nightgown a lot to work, and she always had four rollers in her hair," recalled Marshall, explaining: "It was her thing.
"She was a unique, unique individual and funny as can be. It's a great loss to comedy."
Johnson later wrote for the TV series "It's Garry Shandling's Show" and was a writer and supervising producer on the 1991 series "Good Sports."
She also was one of the writers for the 1979 comedy "Americathon" and the 1982 Jerry Belson-directed comedy "Jekyll and Hyde … Together Again."
Besides her daughter, Johnson is survived by her seventh husband, Charles Lohr; and her brother Gordon Belson.
The family is planning a private memorial service.