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, Mr Johnson promised he would "deliver Brexit, unite the country and defeat Jeremy Corbyn".
Speaking at the Queen Elizabeth II centre in London, he said: "We are going to energise the country.
"We are going to get Brexit done on 31 October and take advantage of all the opportunities it will bring with a new spirit of can do.