Sinatra died of natural causes at her Rancho Mirage, California, home surrounded by family and friends, said John Thoresen, director of the Barbara Sinatra Children's Center.
With her husband's help, Barbara Sinatra founded a nonprofit center in 1986 to provide therapy and other support to young victims of physical, sexual and emotional abuse.
In the years since, Thoresen said, more than 20,000 children have been treated at the center in the desert city of Rancho Mirage and hundreds of thousands more throughout the world through videos it provides.
A former model and Las Vegas showgirl, Barbara Sinatra was a prominent Palm Springs socialite in her own right before she married her husband in 1976 when he was 60 and she 49. They remained wed until his death at in 1998 age 82.
She met the singer through her previous husband, Zeppo Marx of the famous Marx Brothers comedy team. Marx and Frank Sinatra had been close friends and neighbors in Rancho Mirage until she left Marx.
It was her third marriage, Sinatra's fourth and the most enduring union for both.
Frank Sinatra had previously been married to Nancy Sinatra (mother of their children Nancy and Frank Jr.), as well as actresses Ava Gardner, who died in 1990, and Mia Farrow.
Over the years, Frank and Barbara Sinatra played an active role in the children's center.
"Frank would come over and sit and read to the kids," Thoresen said of the sometimes volatile entertainer.
"But the best way she used Frank," he added with a chuckle, "was she would say, 'I need a half-million dollars for this, so you do a concert and I get half the money.'"
She remained active at the center until recently, pushing for creation of the video program just last year and making sure the children had anything they needed, Thoresen said.