Johnson has been elected new Conservative leader in a ballot of party members to become the next UK prime minister.
He beat Jeremy Hunt comfortably, winning 92,153 votes to his rival's 46,656.
The former London mayor takes over from Theresa May on Wednesday.
In his victory speech, Johnson promised he would "deliver Brexit, unite the country and defeat Jeremy Corbyn."
Speaking at the Queen Elizabeth II Center in London, he said, "We are going to energize the country."
"We are going to get Brexit done on October 31 and take advantage of all the opportunities it will bring with a new spirit of can do."
"We are once again going to believe in ourselves, and like some slumbering giant we are going to rise and ping off the guy ropes of self doubt and negativity."
May congratulated Johnson, promising him her "full support from the back benches."
Almost 160,000 Conservative members were eligible to vote and turnout was 87.4%.
Johnson's share of the vote – 66.4% – was slightly lower than that garnered by David Cameron in the 2005 Tory leadership election (67.6%).
Boris Johnson is Britain’s next prime minister.
A sentence that might thrill you. A sentence that might horrify you. A sentence that 12 months ago even his most die-hard fans would have found hard to believe.
But it's not a sentence, unusually maybe for politics, that won't bother you either way.
Because whatever you think of Boris Johnson, he is a politician that is hard to ignore.
With a personality, and perhaps an ego, of a scale that few of his colleagues can match. This is the man who even as a child wanted to be "world king."
Now, he is the Tory king, and the Brexiteers are the court.