"She died comfortably surrounded by family and friends at her home", said John E. Thoresen, Director and Executive Officer of the Barbara Sinatra Children's Center Foundation to the Desert Sun. The facility provides counseling services for abused and neglected children.
Ms Sinatra was a prominent child abuse campaigner, establishing the Barbara Sinatra Children's Centre to help children in California in 1986.
Frank Sinatra and Barbara Sinatra pose with at Caesars Palace in Las Vegas on May 23, 1976.
There's was the longest lasting of the singer's marriages. She later Wednesday Zeppo Marx of the famous Marx Brothers comedy family before her marriage to Frank.
Sinatra used her husband's fundraising prowess to found the Barbara Sinatra Children's Center, which a spokesperson said has allowed thousands of children to benefit from therapy.
Barbara met Frank when he was living next door to the married couple, and they became friends during the tail end of his tempestuous relationship with Ava Gardner, even playing couple's tennis together.
"A very wise French lady once said to me: 'You never worry about old flames".
Shortly after her move, she married Robert Oliver, but they divorced, and she then married Zeppo Marx in 1959.