Corbyn criticized Johnson's campaign platform and questioned the strength of his mandate.
"Boris Johnson has won the support of fewer than 100,000 unrepresentative Conservative Party members by promising tax cuts for the richest, presenting himself as the bankers' friend and pushing for a damaging 'no deal' Brexit. But he hasn't won the support of our country," Corbyn wrote on Twitter.
Before the result, UK Skills Minister Anne Milton resigned, saying she had "grave concerns" about Johnson's willingness to take the UK out of the EU without a deal. Alan Duncan resigned as Foreign Office minister on Monday, saying he refused to serve under Johnson, who he labeled a "circus act".
Further resignations are expected over the coming hours from both International Development Secretary Rory Stewart and Chancellor of the Exchequer Philip Hammond, who have both indicated they do not wish to serve in a Cabinet headed by Johnson.
Johnson entered politics as a member of Parliament in 2001, and later served two terms as Mayor of London. Johnson reentered Parliament in 2016, when he established himself as a leading figure in the "Vote Leave" campaign in the EU referendum.
Johnson was strongly tipped to take over as prime minister when the leave campaign won. However, his political ally Michael Gove dashed his chances by publicly questioning Johnson's ability to lead.
Theresa May then succeeded David Cameron as prime minister and appointed Johnson as foreign secretary. Johnson once again positioned himself as a potential leadership candidate when he resigned over May's handling of Brexit negotiations, paving the way for an ultimately successful run for the office of prime minister.
Donald Trump said on Twitter: "Congratulations to Boris Johnson on becoming the new Prime Minister of the United Kingdom. He will be great!"
