Nancy Reagan, the actress and former first lady to Ronald Reagan, has died at the age of 94, the Reagan Library said.
Mrs Reagan was fiercely protective of her husband Ronald through a Hollywood career, eight years in the White House, an assassination attempt and Alzheimer's disease.
Mrs Reagan, who became one of the most influential first ladies in US history during her Republican husband's presidency from 1981 to 1989, died this morning at her home in Los Angeles of congestive heart failure.
Her husband, who affectionately called her "Mommy" while she called him "Ronnie", died in 2004 after a long struggle with Alzheimer's.
As Nancy Davis, she was a Hollywood actress during the 1940s and 1950s and married Reagan, a prominent film actor, in 1952.
She then served as first lady of California during her husband's stint as California governor from 1967 to 1975 before moving into the White House after his decisive victory over incumbent Democratic President Jimmy Carter in 1980.
Her most publicised project as first lady was the "Just Say No" anti-drug campaign. After her husband developed Alzheimer's disease, she became an advocate for discovering a cure.
She was diminutive and soft spoken but Nancy Reagan's strong will, high-tone tastes and clout with her husband made her a controversial figure during his presidency.
As Reagan's wife, political partner and adviser, she became one of America's most potent first ladies, alongside the likes of Franklin Roosevelt's wife Eleanor; Woodrow Wilson's wife Edith and Bill Clinton's wife Hillary.
With the passing of Nancy Reagan, God and Ronnie have finally welcomed a choice soul home. https://t.co/NjiOBT9xoQ— Mitt Romney (@MittRomney) March 6, 2016
"I see the first lady as another means to keep a president from becoming isolated," she said in 1985.
"I talk to people. They tell me things. And if something is about to become a problem, I'm not above calling a staff person and asking about it.
"I'm a woman who loves her husband and I make no apologies for looking out for his personal and political welfare."
Tiny and frail in her later years, Mrs Reagan devoted her time to caring for her ailing husband at their home in Los Angeles' exclusive Bel Air enclave.
She was always a stickler for protocol and detail and stoically presided over the former president's week-long funeral and celebration of his life in June 2004.
She will be buried next to her husband, at the Ronald Reagan Presidential Library in Simi Valley, California, a statement by the Reagan Library said.
Tributes paid to Nancy
US President Barack Obama and his wife Michelle praised Mrs Reagan as a "proud example".
The Obamas said Mrs Reagan transformed the role of first lady during her White House tenure, and later became an advocate for millions suffering from Alzheimer's disease.
I am saddened by the passing of my step mother Nancy Reagan...She is once again with the man she loved.God Bless...— Michael Reagan (@ReaganWorld) March 6, 2016
Former first lady Barbara Bush said she and her husband, former President George HW Bush, who was vice president under Mr Reagan, took comfort in knowing Nancy Reagan would be reunited with her husband, the late president.
Arnold Schwarzenegger, who like Ronald Reagan rode his Hollywood fame to the governor's office in California, said on Twitter that Nancy Reagan was "one of my heroes".