Reagan married her husband former president Ronald Reagan in 1952. The couple remained together until his death from Alzheimer's disease in 2004.
She will be buried next to her husband at the Ronald Reagan Presidential Library in Simi Valley, according to her spokesperson.
George Shultz served as Secretary of State for most of Reagan's presidency.
Shultz says he spoke to Mrs. Reagan over the phone just a few weeks ago and could tell then that she wasn't doing well.
He says she sounded weak and couldn't really converse, adding that she only gave only one word answers.
Shultz said, "She will be remembered as a beautiful, gracious, support for her husband that turned out to be a really giant president and she was very much a part of it."
The last time Schultz saw Reagan in person was at the Reagan Library in Simi Valley a few years ago.
MORE: Notable figures react to news of Nancy Reagan's death
The public will be allowed to pay their respects to Reagan at the library, but details were not immediately released.
In lieu of flowers, she had requested that people make contributions to the library and its foundation at The public will be allowed to pay their respects to Reagan at the library, but details were not immediately released.
In lieu of flowers, she had requested that people make contributions to the library and its foundation at www.reaganlibrary.com.
As news of Reagan's death hit social media, many paid their respects.
Former first lady Barbara Bush released the following statement:
"Nancy Reagan was totally devoted to President Reagan, and we take comfort that they will be reunited once more. George and I send our prayers and condolences to her family," the statement said.
Former president George W. Bush also released a statement:
"Laura and I are saddened by the loss of former First Lady Nancy Reagan. Mrs. Reagan was fiercely loyal to her beloved husband, and that devotion was matched only by her devotion to our country," it said. "...Laura and I are grateful for the life of Nancy Reagan, and we send our condolences to the entire Reagan family."
Reagan was born on July 6, 1921 in New York City and was raised in Chicago. She started her career as an actress in stage, movies and television, which eventually led to her meeting her husband.
After Ronald Reagan became California governor in 1967, Nancy began visiting wounded Vietnam veterans and began projects focusing on POWs and servicemen missing in action. She would often visit hospitals and homes for the elderly, as well as schools for physically and emotionally handicapped children.
When her husband became president in 1981, Reagan then began focusing on fighting alcohol and drug abuse among the youth with her "Just Say No" campaign.
At the end of Ronald Reagan's presidential career in 1989, Nancy created the Reagan Foundation as a way to continue to educate people about the dangers of substance abuse.
She is survived by her two children Patti Davis and Ronald Prescott Reagan as well as her brother Dr. Richard Davis.
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Nancy Reagan was one of my heroes. She served as First Lady with unbelievable power, class and grace and left her mark on the world. (1/2)— Arnold (@Schwarzenegger) March 6, 2016
She's with her Ronnie now, but those of us she left behind will miss her dearly. (2/2)— Arnold (@Schwarzenegger) March 6, 2016
Nancy is where she has always wanted to be with her Ronnie...Now she is at peace...— Michael Reagan (@ReaganWorld) March 6, 2016