The charismatic Conservative MP played a decisive role in the campaign to lead Britain out of the European Union -- an endeavor many saw as partly an effort to position himself as the future leader of the ruling Conservative Party, and of the country.
The announcement drew a stunned response across the UK and beyond. On social media, it was met with disbelief and anger.
Since the unexpected Leave vote sent the pound tumbling, hurt global markets and inspired renewed talk of Scottish independence, many have seen Britain as a casualty of Johnson's now-thwarted leadership ambitions.
"I am very surprised #BorisJohnson ran the campaign to get us out of the EU and didn't have the guts to re establish the country! Odd move," tweeted entrepreneur Mark Wright.
Wright said Johnson obviously did not truly want a Brexit. "He wanted a close race to show campaign leadership skills for a PM move later on. At what cost to us(?)," he tweeted.
Labour MP Jo Stevens responded with disgust, describing Johnson as "narcissism personified."
"Cameron & #BorisJohnson wrecked the UK. And now neither will take any responsibility," she tweeted.
CNN political contributor Robin Oakley said that "undoubtedly (people are) going to feel let down that he's not standing," adding that he had spoken to many people who had voted Leave due to the campaigning of Johnson, the larger-than-life former journalist.
"Something's gone badly wrong here," Oakley said, referring to the apparent split between Johnson and Justice Secretary Michael Gove, who had campaigned closely together in leading the Leave camp.
In a surprise move ahead of Johnson's announcement, Gove announced that he himself had decided to run for the leadership, after concluding that Johnson "cannot provide the leadership or build the team for the task ahead."
Gove, who was previously education secretary from 2010 to 2015, was the leading Leave campaigner within Cameron's Cabinet.
Johnson's decision means that five Conservative MPs will compete to replace Cameron, who announced his intention to resign after narrowly losing his campaign to persuade voters to remain in the EU in the national referendum last week.
They include three Cabinet ministers: Gove, Home Secretary Theresa May and Work and Pensions Secretary Stephen Crabb. Former defense secretary Liam Fox and Energy Minister Andrea Leadsom are also running.
May: Unity needed
May, Britain's internal affairs chief since 2010, was expected to be the main rival to Johnson for the Conservative leadership, said Oakley. A Euroskeptic who voted Remain, she and Gove now appear to be the frontrunners to be Britain's next PM.
Announcing her candidacy at an event in central London Thursday, she said post-referendum Britain needed "strong, proven leadership to steer us through this period of economic and political uncertainty and to negotiate the best possible terms as we leave the European Union."
"We need leadership that can unite our party and our country," she said.
"With the Labour Party tearing itself to pieces and divisive nationalists in Scotland and Wales, it is nothing less than the patriotic duty of our party to unite and govern in the best interest of the whole country."
Gove had campaigned closely with Johnson for a Brexit, but said he had decided to run himself after concluding that Johnson "cannot provide the leadership or build the team for the task ahead."
Crabb, a Cabinet minister and MP for the Welsh electorate of Preseli Pembrokeshire, announced his leadership ambitions at a news conference on Wednesday.
"On the rainy rugby fields of west Wales I learned that it's not a question of waiting for the ball to pop out the back of the scrum -- if you want it, you do what's required and you get your hands on it," said Crabb, who supported the Remain campaign.
MP Liam Fox, the former defense secretary who resigned from the government in 2011, put his hat into the ring Wednesday, while Leadsom announced her candidacy via Twitter Thursday. Both MPs supported the Leave vote.
"Delighted to say I'm running for the @Conservatives Leadership. Let's make the most of the Brexit opportunities! #FreshStart," she wrote.
Conservative MPs who wished to contest the leadership had until noon local time (7 a.m. ET) to declare their run.
Corbyn in anti-Semitism controversy
The referendum result sent shockwaves through the British political establishment, leaving the leadership of both major parties up for grabs as the country faces an uncertain future.
The opposition Labour Party is also in turmoil, with leader Jeremy Corbyn facing a coup from his own MPs amid criticisms he campaigned poorly to keep Britain in the EU. Corbyn has weathered a mass of defections from his leadership team and overwhelmingly lost a no-confidence vote by Labour MPs Tuesday, but is refusing to resign.
He is expected to face a formal challenge from his MPs once a rival leadership candidate is chosen.
Corbyn walked into a fresh firestorm Thursday when he appeared to compare the Israeli government to ISIS -- at the launch of a report into alleged anti-Semitism within his party.
"Our Jewish friends are no more responsible for the actions of Israel or the Netanyahu government than our Muslim friends are for those of various self-styled Islamic states or organizations," Corbyn said, speaking at the launch of a report commissioned to investigate accusations of anti-Semitism within Labour's ranks.
His comments drew swift criticism in Britain and Israel, with Israeli politicians calling on British Labour MPs to dump the embattled leader.
The inquiry was instigated in response to an anti-Semitism controversy that gripped the party earlier this year, after it was revealed that an MP had shared an image on social media, prior to taking office, which called for Israel to be relocated to the United States.
Wednesday, in the first Parliamentary session since the referendum, Cameron told Corbyn in a fiery exchange that he should step down "in the national interest."
"It might be in my party's interest for him to sit there. It's not in the national interest, and I would say, for heaven's sake man, go."
May: 'No second referendum'
Amid the post-referendum turmoil, members of the public, as well as MPs, have called for a second, do-over referendum before the government formally begins the process of leaving the European Union.
But although she campaigned to remain, May said Thursday that there could be no revisiting the vote.
"Brexit means Brexit. The campaign was fought. The vote was held. Turnout was high, and the public gave their verdict," she said.
"There may be no attempts to remain inside the EU, no attempts to rejoin it through the back door. And no second referendum."