Rosemary, as she was called, was born on September 13, 1918. It became apparent early on that she had special needs. The usual childhood developments of crawling, walking, and speaking occurred much later than usual. Due to her slow development, Rose Kennedy paid particular attention to Rosemary. At first Rose believed that her daughter's slow development was a result of the natural differences in children. When Rosemary was finally tested, however, it was discovered that she was mentally retarded. At the time, there was almost no literature on the condition, nor was there substantial help for caretakers. Most often, it was recommended to place the child in an institution. Nevertheless, Rose and Joe refused to send their daughter away.

Instead, they hired tutors and governesses to care for her. Rosemary learned to read, write, dance, and play tennis, and she was also an active participant in family activities such as sailing. Her participation was in part a reflection of Joe and Rose's desire to keep her condition hidden. Most people, even extended family, were unaware of it. She was presented as the shy, quiet child.

In 1938, when her younger sister Kathleen was presented at court at Buckingham Palace, Rosemary was by her side. Rosemary had spent hours practicing, and when it was all over no one had noticed that she was any different from her sister. In 1939, while Rosemary accompanied her father in England during his ambassadorship, two reporters wrote her a letter requesting an interview. Joe composed a reply for her, and she copied it. In it, she stated that she was studying psychology after having just received a teaching degree. In truth, Rosemary was attending a special school.

By 1942, Rosemary abruptly changed. She had been happy, sweet, and easy to get along with, but suddenly she became prone to violent outbursts. She began screaming, hitting, and swearing. Additionally, she took up the habit of roaming the streets alone in the middle of the night. It was believed that Rosemary was finally feeling the effects of womanhood and that she was frustrated that unlike her siblings, she was unable to accomplish the same level of achievement.

Whatever the reason, Rose and Joe were extremely worried. Joe decided that a prefrontal lobotomy was the best course of action. According to experts, this type of operation would relieve her of the worry she felt but would not interfere with her ability to function. Joe, without telling Rose, had the surgery performed in 1941. When Rosemary emerged, it was clear that something had gone wrong. Her ability to speak was severely impaired and her head, which had once stood erect, tilted to the side.

After the surgery, Joe still told Rose nothing about it. He did inform her that he had placed her in an institution and recommended that she not visit for a while. Rose remained absent from Rosemary's life until Joe had a stroke in 1961. She finally visited her at St. Coletta's, an institution in Jefferson, Wisconsin. It was then that she discovered something was wrong. Not long afterward, she found out the ugly truth of what Joe had done. Rosemary remained in an institution until she died on January 7, 2005.