Scott Forstall, Johnson's son by her first marriage, and The Altenheim, a community for the elderly, confirmed the death, St. Louis Public Radio reported.
Johnson met Masters, a gynecologist, when he hired her as a research assistant in 1957 at Washington University in St. Louis, a year after the end of her first marriage. Masters and Johnson married in 1971 and divorced two decades later.
Masters and Johnson took the research of Alfred Kinsey further. They developed instruments to measure sexual arousal and recruited subjects, about 700 in all, who agreed to masturbate or have sex while their responses were recorded.
The result was the 1966 book "Human Sexual Response," which made Masters and Johnson a household name.
In a 1994 New York Times interview, Johnson said she and Masters had become a brand, that even younger people knew them through college textbooks.
"We are like Kleenex to tissue," she said.
Johnson, who had studied piano and performed on the radio as a country music singer, had no formal medical training. But she learned terminology and method from Masters.
Masters died in 2001.
Johnson is survived by her son and a daughter, both from her first marriage.