She passed peacefully on Sunday with her family by her side, a statement from the Carter Center said.

In May, the Carter family said she had dementia but was continuing to live happily at the couple’s home in Plains, Georgia.

Jimmy and Rosalynn Carter were the longest-married presidential couple, having wed in 1946 when he was 21 and she was 18. After his solo term ended in 1981, he also enjoyed more post-White House years than any president before him, and she played an instrumental role during those years, including as part of the nonprofit Carter Center and the Habitat for Humanity charity.

“The best thing I ever did was marry Rosalynn,” Carter told the C-SPAN cable TV channel in 2015.

“That’s the pinnacle of my life.”

She was seen as unassuming and quiet before coming to Washington in 1977 but developed into an eloquent speaker, campaigner and activist.

Before Jimmy Carter was elected president in 1976, Roslynn was largely unknown outside of Georgia, where her husband had been a peanut farmer-turned-governor. A Democrat, he served one four-year term, losing his 1980 re-election bid to Ronald Reagan, a Republican former California governor and Hollywood actor.

In Washington, the Carters were a team, with the president calling her “an extension of myself” and “my closest adviser.” She was often invited to sit in as an observer at Cabinet meetings and political strategy discussions.

In a 1978 interview with magazine editors, Carter said he shared almost everything with his wife except top-secret material.

“I think she understands the consciousness of the American people and their attitudes perhaps better than do I,” he said.

The First Lady also was sent on important official missions to Latin America and was part of the unsuccessful campaign for ratification of the Equal Rights Amendment to the US Constitution to ensure equal treatment of women under the law.

Eleanor Rosalynn Smith was born August 18, 1927, in Plains to Edgar and Alice Smith, and married Carter on July 7, 1946. They went on to have four children.

Her interest in mental health issues stemmed from the early 1970s when she began to realise, while helping her husband campaign for governor, the depth of the problem in her home state of Georgia and the reluctance of people to talk about it.

As first lady of Georgia, she was a member of a governor’s commission to improve services for the mentally ill.

In the White House, she became honorary chair of the President’s Commission on Mental Health, key to passage of a 1980 act that helped fund local mental health centres.

After leaving Washington she pursued her work through the Carter Center, a private, nonprofit institution the Carters founded in Atlanta in 1982. She continued to advocate for mental health, early childhood immunisation, human rights, conflict resolution and the empowerment of urban communities.

“I hope our legacy continues, more than just as first lady, because the Carter Center has been an integral part of our lives. And our motto is waging peace, fighting disease and building hope. And I hope that I have contributed something to mental health issues and help improve a little bit the lives of people living with mental illnesses,” she told C-SPAN in a 2013 interview.

In their post-Washington years the Carters were also key figures in the Habitat For Humanity charity, helping build homes for needy families. Their humanitarian efforts were crowned in 2002 when Jimmy Carter was awarded the Nobel Peace Prize.