Boris Johnson became Prime Minister of the United Kingdom after winning the Conservative Party leadership election.

Alexander Boris de Pfeffel Johnson is a British politician and writer who served as Prime Minister of the United Kingdom and Leader of the Conservative Party from 2019 to 2022. He was previously Foreign Secretary from 2016 to 2018 and Mayor of London from 2008 to 2016. He was Member of Parliament (MP) for Henley from 2001 to 2008 and for Uxbridge and South Ruislip from 2015 to 2023.

The prime minister of the United Kingdom is the head of government of the United Kingdom. The prime minister advises the sovereign on the exercise of much of the royal prerogative, chairs the Cabinet, and selects its ministers. Modern prime ministers hold office by virtue of their ability to command the confidence of the House of Commons, so they are invariably members of Parliament.

The 2019 Conservative Party leadership election was triggered when Theresa May announced on 24 May 2019 that she would resign as leader of the Conservative Party on 7 June and as Prime Minister of the United Kingdom once a successor had been elected. Nominations opened on 10 June; 10 candidates were nominated. The first ballot of members of Parliament (MPs) took place on 13 June, with exhaustive ballots of MPs also taking place on 18, 19 and 20 June, reducing the candidates to two. The general membership of the party elected the leader by postal ballot; the result was announced on 23 July, with Boris Johnson being elected with almost twice as many votes as his opponent Jeremy Hunt.