It means Johnson looks certain to finally pass a Brexit deal in Parliament on Friday after years of political deadlock, a huge symbolic moment that will ensure Britain leaves the European Union next month.
So Brexit will be "done," right?
The most likely answer is no.
Britain appears headed for at least another 12 months of potentially tortuous negotiations over the terms of its departure from the European Union.
"Actually resolving what Brexit is going to look like going forward, and what that means for businesses and citizens is a whole new task," said Maddy Thimont Jack, a senior researcher at the Institute for Government, a London think tank. "And the British government has a very, very tight time frame in which to do it."
Brexit was always meant to be a two-stage process.
The turmoil since its 2016 Brexit referendum has largely concerned the first phase: the divorce itself. Getting over this hurdle will indeed be a huge symbolic moment.
Whereas his predecessor, Theresa May, suffered a series of humiliating defeats, Johnson's power in Parliament means he will easily pass his Withdrawal Agreement Bill on Friday and should deliver Brexit by the deadline of Jan. 31.
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On Thursday, his fledgling government will present its legislative program in what's known as the Queen's Speech. (The monarch doesn't write it, she just reads it out.)
By ruling out an extension, Johnson spooked financial markets, the pound slipping by as much as 0.7 percent against the dollar Tuesday after its 18-month peak following Johnson's election.
Meanwhile, many in Brussels see the move as one that harms the U.K. more than any of the remaining 27 member states. Countries such as Ireland and France are projected to suffer economically in the event of no-deal — but none more so than the U.K. itself.
There are many unknowns, not least of which is what each side is prepared to concede.
Johnson says he wants a "fantastic" free trade agreement, removing tariffs and other restrictions on goods. "But apart from that, he has been light on detail in terms of what exactly he wants," Jack of the Institute for Government said.
Europe will want concessions in return for access to its market. It's eager to avoid London becoming "Singapore-on-Thames" — outmatching the bloc by retaining European market access while slashing its own domestic regulations.
"The only thing we know is that the U.K. leaving the E.U.," Jack added. "We don't really know anything else at this point."
Alexander Smith is a senior reporter for NBC News Digital based in London.