Benson, who wrote under the pen name Carolyn Keene, authored the initial Nancy Drew mysteries but was not allowed to reveal her true identity until a 1980 court case overturned the secrecy clause in her contract with the publisher.
Benson -- Millie to her friends -- became ill Tuesday afternoon and later was taken from her home by ambulance to Toledo Hospital. She died at 8 p.m., her daughter, Peggy Wirt, said.
Toledo Blade Publisher and Editor-in-Chief John Robinson Block described Benson as "one of the greatest women writers and journalists of the 20th century."
"She was gutsy and daring, a living embodiment of her Nancy Drew heroine," he added. "She influenced generations of Blade reporters. I will never forget her."
Benson worked as a reporter and columnist for 58 years. She retired in January but continued writing a monthly column, despite failing eyesight and diminished hearing.
"Going to work was a way of life for me and I had no other," she wrote in a December column that preceded her retirement.
In addition to her newspaper and fiction-writing careers, Benson was a pilot and adventurer who studied archaeology in the jungles of Central America. In 1927, she became the first woman to receive a masters degree in journalism from the University of Iowa. The Underwood typewriter she used to write the Nancy Drew books rests in the Smithsonian Institution.
Benson was born July 10, 1905, to Dr. J.L. and Lillian Augustine in Ladora, Iowa. She wanted to be a writer at an early age and published her first story in 1919 in the former St. Nicholas Magazine. Her first book won publication while she was still at the University of Iowa, where she financed her education by selling nearly 100 short stories.
Benson submitted a trial manuscript for the Ruth Fielding series to the Stratemeyer Syndicate, which published the Bobbsey Twins, Tom Swift, Rover Boys, Hardy Boys and other series. It was so well received that syndicate owner Edward Stratemeyer offered her the chance to work on a new series of detective stories, featuring a teenager named Nancy Drew.
Benson wrote 23 of the first 25 Nancy Drew mysteries under the Keene name, which she shared with former Boston Globe reporter Harriet S. Adams, one of Stratemeyer's daughters. Adams and her sister decided to rewrite the series in 1959 to make the heroine less of a tomboy.
"I wanted to do something different," Benson once said. "The heroines of girls' books back then were all namby-pamby. I was expressing a sort of tomboy spirit."
Adams died in 1982.
For her work, Benson was paid a flat fee of about $125. She received no royalties and signed away the rights to the stories.
"Millie's innovation was to write a teenage character who insisted upon being taken seriously and who by asserting her dignity and autonomy made her the equal of any adult," Nancy Drew expert Ilana Nash told Wednesday's Toledo Blade. "That allowed little girls to dream what they could be like if they had that much power."
Benson's Nancy Drew offerings sold 100 million copies over seven decades and were translated into 17 languages. They also spawned four movies, a television series and a series of products.
Benson wrote other youth fiction as well, in addition to Cub, Brownie, Girl and Explorer Scout books, short stories and articles for children's magazines. Among her favorite characters was Penny Parker, the heroine of a series she wrote under her own name. Parker was the sleuth-like daughter of a newspaper publisher.
"I always thought Penny Parker was a better Nancy Drew than Nancy is," she said in 1993.
Benson married Associated Press reporter Asa Wirt in 1928 and moved with him first to Cleveland and later to Toledo. Wirt died in 1947 and she married Toledo Times Editor George Benson in 1950. He died in 1959.
Benson stopped writing youth fiction in 1959, saying the teenagers for whom she wrote lived in a world "far removed from drugs, abortion, divorce and racial clash."
"Any character I might create would never be attuned to today's social problems," she said in the 1960s when she turned down an invitation to resume her fiction career.
Benson is survived by her daughter, Peggy, of Logansport, Ind. Private services are planned.