A former first lady, Rosalynn Carter, the closest adviser to President Jimmy Carter during his single term and their four decades thereafter as global humanitarians, has died at the age of 96.

The Carter Center said she died Sunday after living with dementia and suffering many months of declining health. The statement announcing her death said she “died peacefully, with family by her side” at 2:10 p.m. at her home in the rural south Georgia community of Plains. It said a schedule of memorial events and funeral preparations would be released later.

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

The Carters were married for more than 77 years, forging what they both described as a “full partnership.” Unlike many previous first ladies, Ms. Carter sat in on Cabinet meetings, spoke out on controversial issues and represented her husband on foreign trips. Aides to Mr. Carter sometimes referred to her — privately — as “co-president.”

“Rosalynn is my best friend … the perfect extension of me, probably the most influential person in my life,” Mr. Carter told aides during their White House years, which spanned from 1977-1981.

Fiercely loyal and compassionate as well as politically astute, Ms. Carter prided herself on being an activist first lady, and no one doubted her behind-the-scenes influence. When her role in a highly publicized Cabinet shakeup became known, she was forced to declare publicly, “I am not running the government.”

Many presidential aides insisted that her political instincts were better than her husband’s — they often enlisted her support for a project before they discussed it with the president. Her iron will, contrasted with her outwardly shy demeanor and a soft Southern accent, inspired Washington reporters to call her “the Steel Magnolia.”

After leaving Washington, the Carters co-founded The Carter Center in Atlanta to continue their work. Ms. Carter chaired the center’s annual symposium on mental health issues and raised funds for efforts to aid the mentally ill and homeless.

Frequently, the Carters left home on humanitarian missions, building houses with Habitat for Humanity and promoting public health and democracy across the developing world.

In 2015, Mr. Carter’s doctors discovered four small tumors on his brain. The Carters feared he had weeks to live. He was treated with a drug to boost his immune system, and later announced that doctors found no remaining signs of cancer.

Ms. Carter helped her husband recover several years later when he had hip replacement surgery at age 94 and had to learn to walk again. And she was with him earlier this year when he decided after a series of hospital stays that he would forgo further medical interventions and begin end-of-life care.

Jimmy Carter is the longest-lived American president. Rosalynn Carter was the second longest-lived of the nation’s first ladies, trailing only Bess Truman, who died at age 97.