Johnson is UK's next PM
Johnson comfortably defeated Jeremy Hunt in the contest to replace Theresa May among Conservative party members.
He said he would "deliver Brexit, unite the country, and defeat Jeremy Corbyn."
The former foreign secretary will make a victory speech in central London this afternoon.
He will also appoint key members of a new-look Cabinet today.
Johnson's "do or die" pledge to leave the European Union by October 31 sets him on a collision course with parliament.
Parliament has repeatedly voted against a no-deal Brexit.
President Trump congratulated Johnson and said he would be a "great" prime minister.