The Rat Pack star enjoyed an on/off romance with the Some Like It Hot actress until she died from an overdose of barbiturates in 1962.
Leading up to the tragedy, Monroe suffered a number of personal problems and illness - but her passing could have been prevented if Sinatra had intervened after she collapsed at his Cal-Neva Lodge resort in Nevada days before her death.
The Hollywood icon invited Monroe to stay with him at the venue, along with actor Peter Lawford and his wife Pat.
But the holiday turned sour when the fragile-looking actress started taking vitamin shots in front of the other guests, only to later breakdown in her room.
Instead of helping out, Sinatra ordered Monroe to be removed from his estate, according to new book, The Secret Life of Marilyn Monroe.
A security guard for Sinatra's Cal-Neva Lodge tells author J. Randy Taraborrelli, "She opened her purse and pulled out those syringes. I was standing right there with Mr Sinatra and Pat Lawford. Marilyn was very casual about it. She was looking for something else and just pulled them out and put them on the table... Then - and I had never seen anything like this before - she put a small hole at the end of the capsule, and swallowed it. 'Gets into your bloodstream faster that way,' she said."
Valet George Jacobs adds, "Frank Sinatra didn't know what to think about any of it. He was upset, though. He loved Marilyn, yes. But for her to maybe die at Cal-Neva while he was there? That would have been terrible. So he said: 'Get her out of here and get her out of here now.' And that was it. We had to do what he said. I mean, the woman was sick. But as compassionate as Sinatra was, he had a line and she crossed it."