She was hired as a secretary at the university's medical school but soon became the assistant and lover of obstetrician-gynecologist William Masters, then collaborated on a large-scale human sexuality experiment - a subject all but taboo at the time.
The couple became known for a revolutionary therapy that brought couples from across the country with sexual dysfunction, including celebrities, to St. Louis for their two-week program.
Masters had impeccable academic and research credentials in infertility and hormone replacement therapy, but some described him as aloof and austere, often difficult to approach. That's where Ms. Johnson came in.
She had a way of putting people at ease, so much so that with "evangelical-like zeal" she figured out how to get volunteers "to drop their pants in the name of science," said author Thomas Maier, who wrote a 2009 book about the couple.
Ms. Johnson recruited graduate students, nurses, faculty wives, and other participants for what Maier described as the "biggest sex experiment in U.S. history."
The late-hours research, first on the medical school campus and later at a nearby building, shattered basic precepts about female sexuality, including Freud's concept that vaginal - rather than clitoral - orgasm was the more mature sexual response for women.
Ms. Johnson took the case studies and asked the uncomfortable questions. Hundreds of couples, not all of them married, would participate in the observed research.
"Here's a woman without a college degree who helped to revolutionize medicine's understanding of human sexuality, whose therapies are taught in medical schools in the U.S. and around the world," Maier said in a 2010 interview with the Associated Press.
That research was later discussed in their 1966 book, Human Sexual Response. And their 1970 book, Human Sexual Inadequacy, explored a therapy they developed for men and women with sexual problems.
Both books were best-sellers translated into dozens of languages.
At the height of their careers, Masters and Johnson were huge celebrities, the topic of late-night talk-show hosts and on the cover of news magazines.
They married in 1971 and divorced 20 years later, when Masters left her to pursue a sweetheart from his youth. Ms. Johnson never remarried.
Their work had its critics, and it was often frowned upon in some circles in an era when sex was seldom discussed publicly.
"There was a lot of grief," her son said Thursday. "There were threats, things of that kind. She was a very strong woman."
Her attorney, Dave Harlan of St. Louis, echoed those sentiments by describing Ms. Johnson as "strong willed, extremely intelligent, extremely independent."
As Masters' health was in decline in the 1990s, and Ms. Johnson was caring for him, he announced he was divorcing her. Their facility, the Masters & Johnson Institute in St. Louis, closed in 1994 after Masters retired. He died in 2001.
Scott Johnson said his mother also retired in the early 1990s.
But their work still fascinates and resonates. Showtime will debut a TV series in September, Masters of Sex, based on Maier's book.
Along with her son, Ms. Johnson is survived by her daughter, Lisa Young, and two grandchildren.