The former London mayor will immediately march into a head-on collision with the 27 EU leaders and his own parliament should he beat his underdog rival, Foreign Secretary Jeremy Hunt.
His problems will be compounded by an oil tanker standoff in the Gulf with Iran.
Passions are running high and Johnson — who charms and alarms in equal measure — will be accomplishing his life-long ambition of heading Britain at one of its most vulnerable times since World War II.
Britons are still bitterly debating the consequences of their narrow 2016 vote to leave the European Union after 46 years of political and economic ties.
Johnson has vowed to take Britain out by the twice-delayed October 31 deadline — with or without a deal — “do or die, come what may”.
But Brussels refuses to renegotiate the pact it struck with outgoing Prime Minister Theresa May after 17 months of bruising talks.
Her repeated failure to ram that deal through parliament forced her to tearfully announce her resignation as Conservative Party leader on June 7.
That triggered a leadership contest in which fewer than 200,000 paying Conservative Party members took part.
The winner will become prime minister on Wednesday and start appointing a new team after being formally appointed by the queen.
– Razor-thin majority –
The new leader’s next steps will likely impact the fates of future generations in Britain and the immediate health of Europe’s economy.
Some pundits are predicting that the new premier may not last more than a few months in the job.
The pound is trading near a two-year low against the dollar and the euro on fears of a dreaded “no-deal” split that severs trade ties abruptly.
The new prime minister takes over with his precariously thin majority in parliament’s lower House of Commons getting smaller by the day.
It was cut to just two on Monday following the suspension of Charlie Elphicke from Conservative ranks after he was charged with three counts of sexual assault.
Johnson’s seemingly imminent appointment triggered finance minister Philip Hammond and several other ministers to announce their resignations.
Alan Duncan quit as Britain’s Europe and Americas minister on Monday — and launched a challenge to Johnson’s leadership before he even takes office.
He tried to force a test of Johnson’s support in parliament via an emergency debate on Tuesday. It was rejected by Commons Speaker John Bercow.
Retiring Justice Secretary David Gauke said he disagreed with Johnson’s strategy but was willing to give him a chance.
“I think he needs to be given an opportunity to go out there to engage with the European Commission,” Gauke said told BBC radio.
“I won’t vote against my party in a confidence motion.”
Hammond said last week that he might make such a vote in order to block a no-deal.
That could trigger an early election — a prospect that political experts say is becoming increasingly unlikely, even as the possibility of a new Brexit refendum appears remote.
The Britain Elects opinion poll aggregator puts the main opposition Labour party on 25 percent support and the Conservatives on 23.
Eurosceptic populist Nigel Farage’s Brexit Party is polling at 21 percent and the pro-EU Liberal Democrats have 18.
The four-way split appears to reflect the relatively even numbers of those who remain for or against leaving the EU.
Johnson’s domestic battles might have to take a backseat during his first days in office due to a brewing standoff with Iran.
The Islamic republic raised the stakes in its nuclear standoff with the United States by seizing a British-flagged tanker on Friday in the strategic Strait of Hormuz.
Iran’s Foreign Minister Javad Zarif accuses Britain of being an “accessory of the economic terrorism of the US”.
Hunt on Monday branded Tehran’s actions “state piracy” and said Britain was planning a European-led protection force for shipping in the Gulf.