Jean Nidetch, a co-founder of Weight Watchers, places a sign reading "Weight Watchers Sq." in New York’s Times Square in 1973. (Anthony Camerano/AP)

“I was a fat housewife married to an overweight bus driver raising two very overweight kids with a fat group of friends and an overweight poodle,” Jean Nidetch once confessed, describing her life before she started a support group that would become one of the best-known weight-loss programs in the world.

The group was Weight Watchers International. Mrs. Nidetch, the organization’s chief evangelist since it was incorporated in 1963, died April 29 at her apartment in Parkland, Fla. She was 91 and had complications from dementia, said a granddaughter, Heather Nidetch.

Mrs. Nidetch had no formal training as a nutritionist and peddled no revolutionary weight-loss regimen. Instead, she offered many dieters a new way to shed pounds — not in shame and alone, but in companionship forged through common struggle, and with regular meetings where they could share their setbacks and triumphs.

Her insight came from her own years-long battle with weight, first as a chubby child and later as a homemaker in Queens, gorging on cookies at night. She was 5-foot-7 and 214 pounds when, by her account, she hit a low point. A fellow supermarket shopper told her that she looked “marvelous,” then asked when she was due. Mrs. Nidetch was not with child.

In 1961, after repeated failures at fad dieting, she enrolled in a public obesity clinic. The meal plan promoted there — heavy on fish, vegetables and fruit — was appealing and effective. But she described the emotional experience as devastating.

“I was one of the fattest people in the group,” she told the Los Angeles Times. “The leader was a nutritionist who was very thin. She said things like, ‘When I look at a big display of food, I get sick to my stomach.’ When she said that, we all looked at each other. It was a secret look. Nobody in that group got sick looking at food.”

Endeavoring to stick to her diet, Mrs. Nidetch invited six overweight friends to her home, where they formed an impromptu support network whose ranks steadily grew. The members bought a scale for weigh-ins. Mrs. Nidetch developed a rewards system including prizes for weight-loss milestones.

“It seemed to help,” she told the New York Times in 1967. “We discovered that other people hid cookies in the laundry basket.”

In October 1962, Mrs. Nidetch reached her target weight of 142 pounds. News traveled about her skill as a motivator. Two dieters at one session, a garment executive named Albert Lippert and his wife, Felice, encouraged her to form a business.

Together, the Lipperts and the Nidetches founded Weight Watchers — an operation that became an empire with trademarked packaged foods, best-selling cookbooks, summer camps for children, franchises and millions of followers around the world.

Regular member meetings remained a hallmark of the program, which has been compared to Alcoholics Anonymous. Dieters greeted one another with the salutation “See you lighter.” People who were not overweight were called “civilians” because they were not similarly embattled.

Weight Watchers went public in 1968 and celebrated its 10th anniversary with an extravaganza at Madison Square Garden featuring celebrity entertainers such as Bob Hope.

Mrs. Nidetch was herself a celebrity, greeted where she went by cheers of “Jean the Queen!” and “Be lean with Jean!” Merv Griffin and Johnny Carson called her onto their TV programs.

“You take away the handicap of obesity and this person becomes someone else,” she told the Australian paper the Courier-Mail in 1993. “Take a jolly fat man for instance. You talk to him and his heart is breaking. He wants to be thin.”

Jean Evelyn Slutsky, the daughter of a cab driver and a manicurist, was born on Oct. 12, 1923, in Brooklyn.

“I’m sure that my compulsive eating habits began when I was a baby,” she wrote in “The Story of Weight Watchers,” a memoir written with Joan Rattner Heilman. “I don’t really remember, but I’m positive that whenever I cried, my mother gave me something to eat. I’m sure that whenever I had a fight with the little girl next door, or it was raining and I couldn’t go out, or I wasn’t invited to a birthday party, my mother gave me a piece of candy to make me feel better.”

She worked for a company that printed horse racing tip sheets and for the Internal Revenue Service before marrying Mortimer “Marty” Nidetch in 1947. Their courtship, she wrote, consisted largely of eating together. Her wedding dress was size 18.

“We developed a whole act about our size and we were the life of every party,” Mrs. Nidetch wrote, according to the reference guide Current Biography. “People waited for us to say something funny, and we usually did. After all, if you’re fat, you have to make a joke about your weight before somebody else does.”

The Nidetches had three children before divorcing in 1971. One son died in infancy, and another son, Richard Nidetch, died in 2006. Survivors include her son David Nidetch of Boca Raton, Fla.; and three grandchildren. Mrs. Nidetch’s second marriage, to Frank Schifano, also ended in divorce.

H.J. Heinz bought Weight Watchers in 1978 for a reported $71.2 million and sold its diet classes to the private European investment company Artal Luxembourg in 1999 for $735 million.

By then, the outfit had become an international powerhouse.

“In Israel, the Jews and Arabs sit together at our classes,” Mrs. Nidetch said in 1993, “and, you know, they don’t hate each other at all. They’re just interested in what they ate for breakfast.”