Boris Johnson's tenure as Prime Minister of the United Kingdom began on 24 July 2019 when he accepted an invitation of Queen Elizabeth II to form a government, succeeding Theresa May, and ended on 6 September 2022 upon his resignation. Johnson's premiership was dominated by Brexit, the COVID-19 pandemic, the Russian invasion of Ukraine, and the cost of living crisis. As prime minister, Johnson also served simultaneously as First Lord of the Treasury, Minister for the Civil Service, Minister for the Union, and Leader of the Conservative Party.

Johnson defeated Jeremy Hunt in the 2019 Conservative Party leadership election on 23 July 2019, and was appointed prime minister the following day. He re-opened Brexit negotiations with the European Union and in early September he prorogued Parliament; the Supreme Court later ruled the prorogation to have been unlawful. After agreeing to a revised Brexit withdrawal agreement but failing to win parliamentary support, Johnson called a snap general election to be held in December 2019, which the Conservative Party won in its first landslide victory since 1987. During Johnson's premiership, the government responded to the COVID-19 pandemic by introducing various emergency powers to mitigate its impact and approved a nationwide vaccination programme. He also responded to the Russian invasion of Ukraine by imposing sanctions on Russia and authorising foreign aid and weapons shipments to Ukraine.

In the Partygate scandal it was found that numerous parties had been held at 10 Downing Street during national COVID-19 lockdowns, and COVID-19 social distancing laws were breached by 83 individuals, including Johnson, who in April 2022 was issued with a fixed penalty notice. The publishing of the Sue Gray report in May 2022 and a widespread sense of dissatisfaction led in June 2022 to a vote of confidence in his leadership amongst Conservative MPs, which he won. In July 2022, revelations over his appointment of Chris Pincher as deputy chief whip of the party while knowing of allegations of sexual misconduct against him led to a mass resignation of members of his government and to Johnson announcing his resignation as prime minister. Following the July–September 2022 Conservative Party leadership election, Johnson was succeeded as prime minister by Liz Truss, his foreign secretary.

Johnson is seen by many as a controversial figure in British politics. His supporters have praised him for being humorous, witty, and entertaining, with an appeal reaching beyond traditional Conservative Party voters, making him, in their view, an electoral asset to the party. Conversely, his critics have accused him of lying, elitism, cronyism and bigotry. As prime minister, his supporters praised him for "getting Brexit done", overseeing the UK's COVID-19 vaccination programme, which was amongst the fastest in the world, and being one of the first world leaders to offer humanitarian support to Ukraine following the Russian invasion of the country. Within Ukraine, Johnson is praised by many as a supporter of anti-Russian sanctions and military aid for Ukraine. His tenure also saw several controversies and scandals, and is viewed as the most scandalous premiership of modern times by historians and biographers alike.

Theresa May, after failing to pass her Brexit withdrawal agreement through parliament three times, announced her resignation as prime minister on 24 May 2019 amidst calls for her to be ousted. Boris Johnson (a key person in the Vote Leave campaign who had served as the Mayor of London and had also served as Foreign Secretary) had already confirmed at a business event in Manchester days earlier that he would run for Conservative Party leader if May were to resign.

Prior to his state visit to the United Kingdom, US President Donald Trump endorsed Johnson for party leader in an interview with The Sun, opining that he thought Johnson "would do a very good job." In the Conservative Party leadership election, Johnson won all five rounds of voting by MPs, and entered the final vote by Conservative Party members as the clear favourite to be elected. On 23 July, he emerged victorious over his rival Jeremy Hunt with 92,153 votes, 66.4% of the total ballot, while Hunt received 46,656 votes. These results were announced during an event in the Queen Elizabeth II Centre in Westminster.

In his first speech as prime minister, Johnson paid tribute to his predecessor Theresa May, and said "No one in the last few centuries has succeeded in betting against the pluck and nerve and ambition of this country. They will not succeed today. We in this government will work flat out to give this country the leadership it deserves, and that work begins now."

On the day of his announcement as prime minister, Johnson handed the role of Chief Whip to "relative unknown" MP Mark Spencer.