Eunice Kennedy, Joseph P. Kennedy, Bishop James Cassidy, Rose Kennedy, John F. Kennedy, John F. Fitzgerald (Rose’s father) and Ted Kennedy in Hyannis Port, Mass., in 1946. (Associated Press)

Evan Thomas is the author of “Robert Kennedy: His Life” and a forthcoming biography of Justice Sandra Day O’Connor.

Eunice Shriver was the fifth of Joseph and Rose Kennedy’s nine children. She did not fit in with the “golden trio” — Joe Jr., Jack and Kick — who sat at the grown-ups’ table, nor with her younger siblings, who sat at the kids’ table. Gawky, toothy, sickly and painfully thin, she was teased, in the sometimes harsh Kennedy way, as “Puny Eunie.” At the same time, she was willful and determined. She wanted to learn from her more glamorous brothers and sisters. In leather-bound notebooks, she wrote down the way they might enter a room to attract attention or make small talk to charm. To her dominating father, she wrote: “Dear Daddy, I know you are very busy. I also know you are advising everyone else in [the] house on their careers, so why not me?”

Angular and fierce, carelessly dressed in pants and moth-eaten sweaters, she could appear manly. “If that girl had been born with balls, she would have been a hell of a politician,” her father reputedly said of her. No less an authority than Richard Nixon — who, as a young congressman in the 1940s, was a frequent guest at the Georgetown house shared by Jack and Eunice — noted that, after dinner, instead of joining the ladies for social gossip, she would light up a stogie and talk politics with the men.

Eunice was the most ardently religious of the Kennedy children — an “old soul,” said her sister Kick — and also the most caring. It was Eunice who looked after Rosemary, the pretty but “slow” Kennedy sister, taking her sailing and to dances. Increasingly given to tantrums as she became a teenager, Rosemary dangerously wandered off at night into the streets. In 1941, her father dispatched Rosemary to have a prefrontal lobotomy, which was botched, leaving her brain-damaged. In the secretive Kennedy family, Rosemary became a disappeared person, vanished from the newsy letters the clan exchanged. Eunice later said she had no idea where her sister was “for a decade after the surgery.”

Could that really be true? In her discerning, often moving biography, Eileen McNamara writes that “Joe Kennedy’s authority over his children was such that Eunice might have known better than to ask.” Eunice worshipped her father, ignoring, or pretending to ignore, his indiscretions. At the same time, consciously or unconsciously, she rebelled. While the men in the family philandered, Eunice spurned the wooing of her faithful suitor Sargent Shriver for seven years and came close to taking vows of chastity. In 1953, it took a charismatic young priest — Theodore Hesburgh, later president of Notre Dame — to persuade her not to become a nun. (“I told her she did have a vocation, but that her vocation was to be Sargent Shriver’s wife and the mother of his children and to continue to do the work she was doing. I think I resolved it to everyone’s satisfaction,” recalled Hesburgh.)

“Eunice” by Eileen McNamara (Simon & Schuster)

When patriarch Joe suffered a debilitating stroke in 1961, Eunice pushed the family to bring Rosemary back into the light, to humanize her condition, in an article written under Eunice’s byline for the Saturday Evening Post (which still did not disclose the lobotomy). And when Joe died at age 81, Eunice brought Rosemary out of exile, taking her swimming and sailing again at Hyannis Port.

By then, Eunice had become the nation’s leading crusader for the rights of the “retarded,” as the mentally disabled were then known. “Mental retardation was like syphilis,” Eunice recalled. “No one mentioned it in polite society.” She devoted the family’s foundation to the cause and tirelessly, relentlessly lobbied and agitated for government funding, publicity and programs. “Just give Eunice what she wants,” her brother, President John Kennedy, told his aides.

Eunice’s legacy includes the Special Olympics, which had its origins in the back yard of her Maryland estate as Camp Shriver (“Where Everybody Is Somebody”) and, with a significant boost from the city of Chicago, became an international event involving millions of special-needs athletes. It may be an exaggeration to claim, as the author’s publisher does, that Eunice “left behind the Kennedy family’s most profound and lasting legacy” (we might not be here at all if JFK had misplayed the Cuban missile crisis). But McNamara has written a fair-minded, well-reported book. The Shriver children wisely trusted her and opened up Eunice’s papers, allowing McNamara to deliver a sensitive, nuanced portrait.

For all her human sympathy, Eunice was anything but cuddly. If her five children wanted to be comforted, they crawled into bed with their father (who slept in a separate room from his wife). “They definitely had their gender roles mixed up,” said their daughter, journalist Maria Shriver. The country-club-raised Eunice could be a little clueless, once suggesting that juvenile delinquents should take up golf and tennis. She was notoriously hard to work for, by one count going through 21 assistants in four years. She was not exactly timely about paying her bills. Carelessly popping pills for her chronic ailments, “she was a working addict, people would say today,” according to her son Bobby.

At the same time, she was the family member who could be counted on to stand by Kennedy children when they got into trouble, as they sometimes did. And as a confessing Catholic, she practiced what she preached. She would invite delinquent girls from the House of the Good Shepherd in Chicago to live in her family apartment. Her husband, Sargent, recalled: “I’d go to the door, and there would be a woman with a suitcase and a plaintive look on her face. ‘Uh-oh,’ I’d think. ‘Another one of Eunice’s girls.’ ”

Describing life in the Shriver home, McNamara writes: “It was only in retrospect that her own children saw Camp Shriver as something unusual. In the moment, a field full of scores of boisterous children — some in wheelchairs playing catch, others in protective helmets banging their heads against trees — was just life with Mummy.” But what a life.

The Kennedy Who Changed the World

By Eileen McNamara

Simon & Schuster. 383 pp. $28