Channel 4 Racing and CBB star McCririck died aged 79 on Friday morning.
The last pictures of John in January showed the colourful pundit looking frail and thin.
Wife Jenny, whose nickname was “The Booby”, revealed John’s secret battle with lung cancer.
And she claimed his decline in health started after his sacking from Channel 4 Racing after 24 years in 2013.
John’s death came nine months after he claimed on ITV’s This Morning that he “didn’t expect to be alive this time next year”.
“When they decided that they didn’t want John his life kind of fell apart”
Jenny, wife, John McCririck
Looking pale and frail, he blamed his appearance on flu.
Jenny – who was married to the pundit for almost 50 years – said in an interview that doctors had found a cancerous tumour while they were treating John for sepsis in January 2018.
He kept both illnesses a secret to avoid too much attention.
Speaking to The Sun, Jenny claimed her husband had “defied medical science” by beating sepsis.
But the disease left him too weak to fight cancer.
She added: “The cancer was small and you would normally have an op to remove it, but because he was so ill he couldn’t do it.
“They would have killed him if they’d done an operation.”
Describing John’s sacking from Channel 4, Jenny went on: “When they decided that they didn’t want John his life kind of fell apart.”
She said: “John was devastated. He told me that he couldn’t live without it.”
John took the broadcaster to an employment tribunal accusing them of ageism and seeking damages of £3m.
But he lost the case, as witnesses said he had been sacked for being “offensive” and “irritating”.
In 2006, John and Jenny appeared on Celebrity Wife Swap, where they switched partners with Tory MP Edwina Currie and her husband John Jones.