In the book, which hits shelves on October 10 and has been serialized this week in the Daily Mail newspaper - which Johnson also writes for - he recalled the monarch's final days at Balmoral, Scotland.
Johnson formally stepped down just two days before Elizabeth II's death in September 2022, and in the years since, there has been fierce speculation over exactly how she passed away.
"I had known for a year or more that she had a form of bone cancer, and her doctors were worried that at any time she could enter a sharp decline," he wrote in the excerpt.
Johnson's account is the first public indication by a former senior government official as to what the Queen's cause of death might have been. It is listed as "old age" on her death certificate.
Johnson isn't the first prime minister to reminisce about his life, time in office and interactions with the late Queen in an autobiography. Former British leaders Tony Blair, Gordon Brown and David Cameron all did so but only in generalities and without the same level of vivid detail as Johnson.
Buckingham Palace has a policy not to comment on books released about the royal family and as such has not confirmed or denied Johnson's assertions.
Johnson, who served as prime minister between 2019 and 2022, recalls traveling to the royal residence of Balmoral for the customary outgoing audience and resignation. Upon his arrival, he remembers being greeted by the Queen's private secretary Edward Young, who suggested to him that she had deteriorated significantly over the summer.
Thinking back on that last time the pair sat together in the Queen's drawing room, Johnson said that he understood Young's forewarning.
"She seemed pale and more stooped, and she had dark bruising on her hands and wrists, probably from drips or injections," he wrote.
"But her mind - as Edward had also said - was completely unimpaired by her illness, and from time to time in our conversation she still flashed that great white smile in its sudden mood-lifting beauty."
Johnson described the weekly prime minster audiences with the monarch as "a privilege" and "a balm."
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"She radiated such an ethic of service, patience and leadership that you really felt you would, if necessary, die for her," he continued. "That may sound barmy to some people (and totally obvious to many more), but that loyalty, primitive as it may appear, is still at the heart of our system.
"You need someone kind and wise, and above politics, to personify what is good about our country. She did that job brilliantly."
The late Queen never shared private medical details with the public. Aides within the royal household still maintain that family members have the same right to medical privacy as anyone else.
King Charles III and Catherine, Princess of Wales have bucked the trend and been more open about their health. The two have shared details about their own cancer diagnoses and recoveries.
However, in both instances, they chose not to divulge the specific form of cancer each has been battling. When pressed, aides said they wanted to share their experiences to raise awareness of the disease.
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