At the time, Lawford was married to Pat Kennedy, JFK's younger sister.
Sometime after her split with Arthur Miller, Marilyn began dating Frank Sinatra and became an unofficial member of Sinatra's "Rat Pack," that group of show-business cronies with whom the legendary singer maintained close personal and professional ties.Core members of the Rat Pack included Dean Martin, Sammy Davis, Jr., Peter Lawford, and Joey Bishop.Marilyn had known Sinatra for many years, and some biographers speculate that the two may have enjoyed a relationship years earlier, though no hard evidence exists to support this.Their friendship was probably renewed during the shooting of The Misfits, when Marilyn was flown to Los Angeles after her breakdown.(The singer would later sell the business, when his link with organized crime was leaked to the press and the public.) Sinatra gave Marilyn a small white poodle to replace the dog she lost in the divorce with Miller.Marilyn, who always had a spirited sense of humor, called the dog "Maf," which was short for Mafia.
Marilyn had known actor Peter Lawford since her starlet days, when he had escorted her to a few Hollywood functions.
Her association with both Sinatra and Lawford undoubtedly brought her into contact with John Kennedy, perhaps as early as July of 1960, when the young senator clinched the Democratic nomination for president.
Supposedly, Sinatra called to inquire about her health and wish her well.
Earlier, he had invited the cast of The Misfits to watch him perform at the Cal-Neva Lodge.
Sinatra was in the process of purchasing the lodge, which was located near Lake Tahoe, directly on the border of California and Nevada.
Marilyn would visit the lodge several times over the remaining two years of her life.