LONDON—British Prime Minister Boris Johnson won a decisive majority in Thursday’s general election, a stunning victory for the Brexit cheerleader that paves the way for the U.K. Parliament to trigger a long-delayed split with the European Union.

Mr. Johnson’s Conservatives secured a majority in Britain’s 650-seat House of Commons, the party’s strongest performance at an election since 1987. With one seat yet to be declared, the party had won 364 seats, 79 more than all the others combined.

The...