Lord Patten, who supports the UK remaining in the EU, said Mr Johnson, a prominent Leave campaigner, "just makes it up as he goes along".
He also said it would be "very difficult" for David Cameron to remain leader if the UK voted to leave the EU.
Vote Leave said it was saddened at "the level of debate" from the Remain camp.
Mr Johnson has previously said voters want proper debate and to hear about the issues, "not personal attacks".
The UK decides whether to stay in or leave the European Union in a referendum on 23 June.
The Conservative Party is deeply split over the the issue, with David Cameron and most of his cabinet campaigning for a Remain vote but about half of the MPs supporting an EU exit.
The debate inside the party has turned increasingly acrimonious, with former deputy prime minister and pro-Remain campaigner Lord Heseltine recently accusing Mr Johnson of losing his judgement over the EU.
Lord Patten, Tory party chairman from 1990 to 1992, added to that criticism of the former London mayor, in an interview with Newsnight's David Grossman on Friday.
"Boris just makes it up as he goes along, and you come across people like that," he said.
"There's a sense in which you can't call Boris a liar. I think he's one of those people in life who simply doesn't really understand the difference between fact and fiction."
He added: "If he can make a good joke by saying something, or if he can write a newspaper article by referring to Hitler and the European Union, he does it - and he doesn't think about it.
"He'll be saying the opposite in a few months' time."
Lord Patten, a European commissioner from 2000 to 2004, said Mr Johnson was an "amiable" character, but added: "Boris is Boris. Tommy Cooper was Tommy Cooper."
In response, Vote Leave said: "It's sad to see the level of debate the 'Remainers' have been reduced to. They were wrong about the euro then and they are wrong now."
And speaking to BBC Newsnight, Leave campaigner Jacob Rees Mogg suggested that Lord Patten's "anger is almost certainly synthetic".
The MP suggested Lord Patten was trying to "bash down" Mr Johnson because he knows that the former London mayor was "hugely popular and trusted" and was making the case for an EU exit "exceptionally effectively".
The Daily Mail reported earlier this week that "senior party figures" had warned of a vote of no-confidence in Mr Cameron after 23 June, whatever the outcome, over his handling of the referendum campaign.
On the question of re-uniting the Conservative Party after the referendum, Lord Patten said that if the UK voted to Leave the EU it would be "very difficult" for Mr Cameron to stay on as leader.
"Are people like Michael Gove and Mr Johnson and Iain Duncan Smith... are they all going to require Mr Cameron to go cap in hand around Europe for the next few years trying to negotiate new trade deals and new agreements with the EU?
"I don't know," he said.
Lord Patten said if Britain voted to remain in the EU, Mr Cameron would have to assert his authority and try to bring the party together.
But he suggested the PM should give Mr Johnson a challenging cabinet post, such as running the Department for Health.
"That will really test whether there is something behind all the jokes," he said.
Lord Patten also criticised the wider Vote Leave campaign, saying it's objective appeared to be "to keep the turnout as low as possible and to tell young people to stay at home and not vote", which , he said, was "extraordinary".
And he added: "I worry that for a lot of the Brexiteers, in a sort of spittle-flecked way, this is a neverendum, not a referendum."
Also in the interview, the former Tory party chairman conceded that he had made the wrong call on Britain joining the Euro.
"I put my hand up," he said. "Everybody occasionally makes a mistake and I think I was wrong about that."
Boris Johnson does not seem to understand "the difference between fact and fiction", former Tory chairman Lord Patten has told BBC Newsnight.