The ex-Conservative leader, 60, met the monarch in September 2022 at Balmoral Castle in Scotland - two days before she died there at her beloved home.
Johnson described his final meeting with the Queen in his upcoming memoir Unleashed.
"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.
Johnson noticed the Queen 'seemed pale and more stooped' at their final meeting at Balmoral. (AP PHOTO)
Before his final meeting with Her Majesty, Johnson said the Queen's private secretary Edward Young warned him her health had "gone down quite a bit over the summer".
"She seemed pale and more stooped, and she had dark bruising on her hands and wrists, probably from drips or injections," he said in his book, which is being serialised by the Daily Mail.
"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 also said the Queen had "known all summer that she was going" but was "determined to hang on and do her last duty" by overseeing the "peaceful and orderly transition" of power from him to his prime minister successor Liz Truss.
Johnson formally resigned from the prime minister's job during his meeting with the Queen, after becoming the 14th British leader of her reign.
He also tells in his book the final piece of advice she gave him, which he said was: "There's no point in bitterness."
"And amen to that. If everyone in politics – and life – could see that as clearly as she did, the world would be a much, much happier place."
Royal author Gyles Brandreth, 76, has also written about how the Queen had a rare form of myeloma – a bone marrow cancer – before her death in his 2022 book Elizabeth: An Intimate Portrait.
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