The cause of death was congestive heart failure, according to her rep Joanne Drake, a spokeswoman with the Reagan Library.
"Mrs. Reagan will be buried at the Ronald Reagan Presidential Library in Simi Valley, California, next to her husband, Ronald Wilson Reagan, who died on June 5, 2004," Drake wrote in a statement.
"Prior to the funeral service, there will be an opportunity for members of the public to pay their respects at the Library."
"It is a very sad day," former Ronald Reagan Chief of Staff Ken Duberstein told NBC News. "Every time she was in the room, he was better, and every time he was in the room she was better. She brought a sense of class and dignity and elegance that everybody admired."
Her step-son Michael Reagan posted on Facebook: "She is once again with the man she loved. God Bless."
"We remain grateful for Nancy Reagan's life, thankful for her guidance, and prayerful that she and her beloved husband are together again," President Barack Obama and First Lady Michelle Obama said in a statement.
Barbara Bush, another influential former first lady, said in a 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."
Reagan was a Girl Scout and named honorary president of the Girl Scouts as first lady. The Girl Scouts of America said in a statement that Reagan would be remembered for her "courage, confidence and character."
Nancy Reagan was born in New York City and when she was six her mother, Edith, a stage actress, married Dr. Loyal Davis, a neurosurgeon. The doctor adopted Nancy and she grew up in Chicago. She later attended Smith College in Massachusetts, where she majored in theater.
Reagan was an actress when she met Ronald Reagan in the early 1950s, then an actor and president of the Screen Actors Guild according to a White House Bio.
She needed to talk to him because her name had been wrongly included on a published list of suspected communist sympathizers. They discussed it over dinner, and she later wrote that she realized on that first blind date "he was everything that I wanted."
They wed on March 4, 1952. Daughter Patti was born in October of that year and son Ron followed in 1958. Reagan already had a daughter, Maureen, and an adopted son, Michael, from his marriage to actress Jane Wyman.
Their marriage lasted 52 years, until Ronald Reagan's death in 2004.
While Ronald Reagan was governor of California from 1967 to 1975, Nancy Reagan worked with numerous charitable groups, and spent hours visiting veterans, the elderly, and the emotionally and physically handicapped.
"The movies were custard compared to politics," Mrs. Reagan once said.
When her husband became president of the United States, First Lady Reagan continued her interest in these groups continued, and arguably became best known for her "Just Say No" program fighting against drug abuse among youth.
When Ronald was shot in 1981 by a would-be assassin, Nancy rushed to his side immediately, and later endured his nearly decade-long battle with Alzheimer's disease.
In recent years she broke with fellow Republicans in backing stem cell research as a way to possibly find a cure for Alzheimer's.
In announcing his Alzheimer's diagnosis in 1994, Reagan wrote, "I only wish there was some way I could spare Nancy from this painful experience."