His convincing victory catapults the United Kingdom towards a showdown with the EU and towards a constitutional crisis at home, as British lawmakers have vowed to bring down any government that tries to leave the bloc without a divorce deal.
Johnson, the face of the 2016 Brexit referendum, won the votes of 92,153 members of the Conservative party, almost twice the 46,656 won by his rival, Foreign Secretary Jeremy Hunt.
May will leave office on Wednesday after going to Buckingham Palace to see Queen Elizabeth, who will formally appoint Johnson before he enters Downing Street.
“We are going to get Brexit done on Oct. 31, and we are going to take advantage of all the opportunities it will bring in a new spirit of ‘can do’,” Johnson, 55, said after the result was announced.
“Like some slumbering giant, we are going to rise and ping off the guy-ropes of self-doubt and negativity.”
Johnson said the mantra of his leadership campaign had been to “deliver Brexit, unite the country and defeat (opposition Labour leader) Jeremy Corbyn - and that is what we are going to do”.
Theresa May announced her resignation last May, after conservative MPs pressed her for failing to get the UK out of the European Union on March 29.