Boris Johnson will replace Theresa May as prime minister after winning the leadership of the Conservative Party on Tuesday.
Johnson won the votes of 92,000 members of the Conservative Party.
This was almost twice as many as his rival, Foreign Secretary Jeremy Hunt.
Johnson is pro-Brexit and has promised to lead Britain out of the European Union.
May will leave office on Wednesday after going to Buckingham Palace to see Queen Elizabeth, who will formally appoint Johnson.
US President Donald Trump quickly tweeted his congratulations, saying: “He will be great!”.