Lillian Carter, the 85-year-old mother of former President Jimmy Carter, died of cancer yesterday in Americus- Sumter County Hospital in Americus, Ga., where she had been hospitalized since Tuesday.

The 39th President was at his mother's side when she died shortly after 5 P.M., as were Mrs. Carter's two other living children, Billy Carter and Gloria Spann, and Jimmy Carter's wife, Rosalynn. Mrs. Carter's other daughter, Ruth Carter Stapleton, died of cancer Sept. 26.

Her physician, Dr. Paul Broun, said Mrs. Carter died of cancer that had spread to the bone. On June 29, 1981, Mrs. Carter underwent a modified radical mastectomy after a malignant tumor was discovered in her left breast.

Mrs. Carter, known to her neighbors as Miss Lillian, was a nurse, a Peace Corps volunteer, an unofficial ambassador and a supporter of civil rights and women's causes. She also said what was on her mind.

In 1977 she became the first woman to receive the Covenant of Peace Prize of the Synagogue Council of America for her contributions to the ''furtherance of international understanding, justice and peace.''

The next year she journeyed to Rome to receive the Ceres Medal of the Food and Agriculture Organization, a specialized United Nations agency, going on to West Africa to study the drought there.

Jimmy Carter called her ''the most influential woman in my life.''

She called him ''just an ordinary little boy,'' adding: ''Actually, his sister Gloria is the most intelligent one in the family.''

Mrs. Carter undertook a number of foreign trips in her son's behalf, including one to Israel in December 1978 for the funeral of Golda Meir, the former Israeli Prime Minister. At Ben-Gurion International Airport, Mrs. Carter noted that Mrs. Meir had said that she wanted no eulogies, then raised her eyes and said:

''Golda, this is not a eulogy to you. This is just me telling you how I wish I had known you.''

Fourth Child in Her Family

Born Aug. 15, 1898, in the southwest Georgia town of Richland, Bessie Lillian Gordy was the fourth of nine children of James Jackson (Jim Jack) Gordy, the local postmaster, and Mary Ida Nicholson Gordy. The family also included two adopted children.

The Gordys moved to Plains, Ga., in 1921, where, against her family's objections, Lillian began nurses' training at Wise Hospital. ''I always wanted to be a nurse,'' she told The Ladies' Home Journal in 1977. ''I guess if I had my life to live over, I would realize that what I really wanted to be was a doctor.''

On Sept. 25, 1923, after completing her training in Atlanta, Miss Gordy married James Earl Carter, known as Earl, who managed the local farm supply store. James Earl Carter Jr. was born on Oct. 1, 1924.

In 1927 the family bought a 700-acre farm in Archery, three miles outside Plains, and later expanded it and acquired a fertilizer bagging and selling plant, a peanut warehouse and a general store for their black farmhands.

Mrs. Carter worked as a nurse. ''I do believe in working women,'' she told Ms. Magazine in 1976, ''and I feel so strongly that a child is better off not to have the mother every minute of the time.''

To the astonishment of her white neighbors, Mrs. Carter's work included caring for black patients. And occasionally she received black visitors in the parlor.

'I Was Born a Liberal'

In 1964 she was co-chairman of President Johnson's campaign in Sumter County. ''People hated Johnson down here because of his stand on civil rights,'' she recalled years later, ''and it got very ugly.''

''People say I was kind to all blacks, which I was,'' she said. ''I was born a liberal. My father, whom I adored, was just as kind to blacks as to whites. It was just a normal thing to him. He didn't talk about it.''

''Now my husband, he was not an integrationist. Earl was of his time. Of his time. He was not like me, certainly, but he did not stop me from doing what I wanted to do.''

In the late 1940's the family moved back to Plains and James Earl Carter Sr. was elected to the Legislature. In 1953 he died of cancer, and his son Jimmy was called home from the Navy to take over the family businesses.

First ''bitter,'' then ''bored'' as a widow, Mrs. Carter consulted a friend and decided to take a job as housemother at the Kappa Alpha fraternity at Auburn University in Alabama. She stayed until 1961, when she returned to Plains. Shortly thereafter, she was back at work, managing a nursing home in Blakely. But she left after two years, complaining that most of the patients were younger than she was.

Drawn by Ad for Peace Corps

In 1966, when her son was making his first bid for the Georgia Governor's office, an unsuccessful one, Mrs. Carter, then 67, saw a television commercial for the Peace Corps that proclaimed: ''Age is no barrier.'' She volunteered, and asked to go to India, saying, ''I wanted to work with people who were underdogs.''

She began her service in Vikhroli, 15 miles north of Bombay, at the family planning clinic of Godrej & Boyce Manufacturing, makers of steel furniture. There, she taught methods of contraception to male workers and assisted with vasectomies.

Discouraged at the slow progress of her work, she asked to be transferred to the company's clinic where ''I did everything but fill prescriptions.'' ''I did dressings, injections, helped the doctor examine and diagnose,'' she said.

Forbidden to treat anyone but employees, she dipped into clinic supplies of bandages and drugs to give to the poor; eventually, company officials agreed to distribute supplies at her expense and she solicited contributions from home.

Back to U.S. in 1968

Her tour of duty over in 1968, she returned to the United States, saying that her time in India ''meant more to me than any other one thing in my life.''

After Jimmy Carter won the 1976 Democratic Presidential nomination, his mother advised him ''to quit that stuff about never telling a lie and being a Christian and how he loves his wife more than the day he met her,'' and, further, to pick a ''good-looking'' running mate. While she made hundreds of campaign speeches for her son, her main contribution was baby-sitting for his 8-year-old daughter, Amy, whom she called ''my heart.''

''I'm a Christian but that doesn't mean I'm a long-faced square,'' Mrs. Carter told an interviewer in 1976. She smoked an occasional cigarette and played bridge for money. She also made no secret of her fondness for bourbon. ''I know folks all have a tizzy about it, but I like a little bourbon of an evening,'' she said. ''It helps me sleep. I don't much care what they say about it.''

Avid Reader of Mysteries

Mrs. Carter was an avid reader and favored mysteries. While in India she opened a package from home and, she said, ''There was my darling Mickey Spillane looking at me from the back of his book.''

She was also devoted to soap operas (her favorites were ''The Young and the Restless'' and ''All My Children'') and to baseball. She was a staunch Dodger fan, beginning in 1947 with the arrival in Brooklyn of Jackie Robinson, the first black major league baseball player. She was also known to leave Plains to see a wrestling match in Columbus.

At her ''pond house'' in Plains, a gift from her children when she returned from India, she liked to take long walks and fish for bass.

In addition to three of her children, Mrs. Carter is survived by 15 grandchildren and eight great-grandchildren.

Graveside services are scheduled for tomorrow at 3 P.M. at Lebanon Cemetery in Plains, according to a spokesman for the hospital. The family has requested that flowers not be sent.