In response, Mr Johnson told parliament he took "full responsibility" but insisted decision-making during the pandemic had been "appallingly difficult" and the government "acted throughout with the intention to save life.in accordance with the best scientific advice".
A mass vaccination campaign that started in December has brought infections and fatalities down sharply, but the government acknowledges it will have to answer serious questions at a future public inquiry.
"The truth is that senior ministers, senior officials, senior advisors like me, fell disastrously short", he told a House of Commons committee.
McNamara, according to Cummings, told the prime minister that there was no plan to fight the virus.
Mr Cummings also attacked Boris Johnson, describing his former boss as "unfit for the job" and claiming he had ignored scientific advice and wrongly delayed lockdowns.
At the center of that disastrous response, Cummings said, was Johnson - who believed the virus was much ado about nothing, comparing it derisively to swine flu.More news: Manhattan district attorney convenes special grand jury in Trump probe
"Lots of key people were literally skiing in the middle of February", he said, including Johnson, who "went away on holiday for two weeks".
But he blasted senior United Kingdom officials for failing to grasp the seriousness of the situation. Reflecting on the failure to close the borders in March 2020, Mr Cummings told MPs that the policy was "madness" because travellers were still arriving from infected areas.
He said officials were guilty of "catastrophic" group-think, pursuing a haphazard strategy of so-called herd immunity before belatedly abandoning it when the likely death toll became clear.
Cummings - who resigned in November 2020 - relayed comments made by deputy cabinet secretary Helen McNamara, who Cummings referred to as "the second most powerful official in the country".
"It's true that I hit the panic button (in March 2020) and said we've got to ditch the official plan..."
It was "like a scene from "Independence Day" with Jeff Goldblum saying 'The aliens are here and your whole plan is broken, '" Cummings said.More news: Italian cable auto crash kills nine, injures two children
Cummings made headlines in May 2020 when newspapers revealed he had driven 250 miles (400 kilometers) across the country after contracting COVID-19, despite a nationwide stay-at-home order.
Cummings' seven hours of testimony to Parliament's science and health committees, who are investigating Britain's pandemic response, gave a dramatic taste of what might come out.
And he repeatedly singled out Health Secretary Matt Hancock for extraordinary criticism, alleging he lied to colleagues on numerous occasions and could have been fired for "at least 15 or 20 things".
"We absolutely reject Mr Cummings' claims about the Health Secretary", the spokesman said.
Meanwhile, Johnson defended the government's response Wednesday, saying "to deal with a pandemic on this scale has been appallingly hard".
During his evidence, Mr Cummings said security concerns around his London home had been an additional, previously undisclosed, factor in the decision.More news: India's coronavirus death toll passes 300,000
He says at the time the government "did everything that they could, in the national interest as they saw it, with the imperfect information that was available at the time".