The Labour leader focused much of his questions on Mr Johnson's previous pledge that no one would have to sell their own home to pay for social care, a promise Sir Keir said has not been kept under the government's funding plan.
Sir Keir said under the plan, "a person with assets worth about £100,000 - most of it tied up in their home - would have to pay £80,000, they'd lose almost everything.
"How on earth does the prime minister think that they can get their hands on that kind of money without selling their home?"
He added: "It's another broken promise, just like he promised that he wouldn't put up tax, just like he promised 40 new hospitals, just like he promised a rail revolution in the North.
"Who knows if he'll make it to the next election, but if does how does he expect anyone to take him and his promises seriously?"
Mr Johnson insisted people living in their home will be protected from selling it to fund their care, as it would not be counted as an asset in the government's assessment of how much a person should contribute - and he defended his £96 billion rail plan, saying there's been "nothing like it for a century".
Sir Keir said it "isn't just broken promises. It's also about fairness. Under his plan someone with assets of about £100,000 will lose almost everything yet someone with assets of about a million pounds will keep almost everything".
He labelled the plan a "classic con game, a Covent Garden pick-pocketing operation - the prime minister is the front man".
SNP Westminster leader Ian Blackford continued the attack, as he pointed to an apparent lack of confidence in Mr Johnson from his own backbenches.
“It used to always be said that the Tory MPs were behind the Prime Minister but my goodness look at the gaps. The rebellion has clearly started," he said.
It's been a tough few weeks for the prime minister with questions raised over his authority - first over alleged corruption in the Tory Party, then what critics say were broken promises on rail and social care, before his bizarre CBI speech which left people asking if he was okay.
He was heavily criticised for his handling of the Owen Paterson debacle, which saw him order a majority of Tory MPs to vote for an amendment which would allow the disgraced former MP to appeal his recommended punishment for breaking lobbying rules.
Opponents said Mr Johnson was trying to rip up the standards rule book to get one of his friends "off the hook" - what followed was a screeching U-turn in which the government reversed its amendment and promised to tighten lobbying rules. Amid the backlash Mr Paterson resigned as an MP.
Then there was a furious reaction in the North of England as Mr Johnson announced the eastern high speed rail (HS2) leg had been scrapped, despite his promise in the run up to the 2019 general election that the line would be laid in order to level up the north and connect it with London.
Days later the government was accused of being disingenuous over its announcement that the cap on the cost of social care would be capped after additional detail was added to the policy which watered down the protection for the less well off compared to the more wealthy.
After weeks of struggling to brush away criticism, his calamitous speech to the CBI could not have came at a worse time.
In the bizarre address to business leaders he made car engine noises, repeatedly lost his place, paused for almost 30 seconds as he attempted to get back on track, before appearing to lose it entirely and talking about his recent trip to Peppa Pig World.
Sir Keir capitalised on the PM's torrid time as he referenced a report from the BBC which said Tory backbenchers find him "embarrassing" and senior people in Downing Street feel his leadership is "just not working".
"Is everything okay prime minister?" he asked in a nod to a query he faced from a reporter following his speech.
Ministers and the PM's spokesman have been forced to defend the PM following the speech, insisting he is on "good form".
Downing Street insisted the PM was "well and he is focused on delivering for the public" following the speech.
"The prime minister briefly lost his place in a speech," his official spokesman said. "He has given hundreds of speeches. I don't think it's unusual for people on rare occasions to lose their place in a speech."
His spokesman added: "The Prime Minister is very much focused on delivering for the public."