Rosalynn Carter, the former first lady of the United States and the closest adviser to Jimmy Carter during his one term as president, has died at age 96.

The Carter Center on Sunday said she “died peacefully, with family by her side” at her rural Georgia home of Plains after living with dementia and suffering many months of declining health.

“Rosalynn was my equal partner in everything I ever accomplished,” Carter said in a statement.

“She gave me wise guidance and encouragement when I needed it. As long as Rosalynn was in the world, I always knew somebody loved and supported me.”

Jimmy and Rosalynn Carter were the longest-married presidential couple, having wed in 1946 when he was 21 and she was 18.

After his 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.

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

Her abiding passion, which carried far beyond her White House years, was for those living with mental illness, not because of any personal connection but because of a strong feeling that advocacy was needed.

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, DC, 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.

The Iranian hostage crisis – in which American diplomats and others were held captive in Tehran after the Islamic revolution – occurred when Carter was seeking re-election. The crisis contributed to the downfall of his presidency as he refrained from campaigning while trying to resolve the standoff.

During that time, Rosalynn Carter sought to support her husband by speaking in 112 cities in 34 states during a 44-day tour.

Her speeches and forays into crowds were credited with helping Carter defeat Democratic challenger Ted Kennedy in the 1980 primaries, although he went on to lose overwhelmingly to Reagan in the general election.

Her interest in mental health issues stemmed from the early 1970s when she began to realise the depth of the problem in 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.