Writing on his Substack blog, Mr Cummings also claimed he knew that Mr Johnson was “unfit to be PM” because he’d admitted it.
Mr Cummings wrote: “On 24 June 2016 in Vote Leave HQ, just after Cameron had resigned, Boris pulled me into the odd little room where the ‘campaign within the campaign’ was run. What now?
“Boris told me with a laugh, ‘Obviously it’s ludicrous me being PM – but no more ludicrous than Dave (Cameron) or George, don’t you think?’
“I agreed and reminded him of the main elements of the deal we’d agreed with (Michael) Gove about what to do next.”
Mr Cummings left Downing Street in November last year following a furious internal power struggle with the Prime Minister’s then-fiancee, Carrie Symonds.
Following weeks of criticism of Mr Johnson, Mr Cummings wrote that he had been questioned about why he took up the Downing Street role “if you knew Boris was so hopeless” and why he was involved in the 2019 general election.
Mr Cummings also claimed the Prime Ministers time as foreign secretary provided “a severe warning” of the dangers of allowing him into Downing Street.
He explained: “Boris won’t read the papers. He cannot chair meetings to save his life.
"He has no idea how Whitehall works and has no interest in it. He wants to believe everyone loves him and is blind to official manoeuvring even when it is brazen…
“His whole being wants to throw himself into the arms of polite young Balliol men and enjoy the trappings of power while they do the work and pull the strings.
“He believes in the system — after all it’s made him famous and is making him PM!”
Despite this, Mr Cummings said he decided to “roll the dice” and support efforts to get Mr Johnson into Number 10, over fears of former Labour leader Jeremy Corbyn becoming prime minister and the possibility of a second EU referendum.