The ex-foreign secretary and London mayor Mr Johnson, 54, confirmed he will stand as a candidate, telling business leaders, "Of course I'm going to go for it".
The Independent has scoured the voting record to see what it reveals about the kind of prime minister Mr Johnson would be.
The Iraq War
Mr Johnson supported military intervention in Iraq in 2003, backing prime minister Tony Blair's efforts in several key votes.
He voted against seeking the support of the UN Security Council and the support of a vote of MPs before committing British armed forces to the conflict and in favour of all means necessary to destroy Iraq's weapons of mass destruction.
Mr Johnson also backed David Cameron's failed attempt to bring about air strikes in Syria in 2015.
Mr Johnson repeatedly voted against a ban on fox hunting in the early 2000s, when he was MP for Henley.
As London mayor, he suggested introducing hunting in the capital after his cat was mauled by a fox, saying: "This will cause massive unpopularity but I don't care."
However, Mr Johnson used his Telegraph column last year to demand foreign aid money is spent on conservation.
He was also spotted recently at a fundraiser for animal charities with his girlfriend Carrie Symonds, who it outspoken on conservation and animal rights.
Mr Johnson is reasonably positive on equality issues, voting in favour of civil partnerships and to repeal a ban on the "promotion" of homosexuality in schools.
He was absent for several key votes on whether gay couples can adopt children.
Mr Johnson was London mayor when gay marriage was legalised, so he did not have a vote.
Mr Johnson voted against measures to prevent climate change, such as setting a carbon reduction target for the UK in 2016.
He also voted against requiring the energy industry to adopt a strategy for carbon capture and storage in the same year.
A keen cyclist, Mr Johnson was associated with the introduction of the cycle hire scheme during his time as mayor of London, which became known as "Boris bikes".
In April, he said he was sympathetic to the aims of Extinction Rebellion but described the young climate change activists as "smug" and told them to "lecture" China instead.
Taxes and benefits
Mr Johnson has consistently voted to raise the income tax threshold to allow lower taxes for higher earners.
He also voted against higher taxes on banks, backed the reduction of capital gains tax and supported restriction of trade union activity.
Due to his tenure in City Hall, Mr Johnson was not involved in the austerity measures associated with Mr Cameron's premiership.
In recent years, he voted to reduce the household benefits cap and for cuts to universal credit for those in work.
Mr Johnson tends to vote for stronger immigration enforcement and for a stricter asylum system.
He voted against banning the immigration detention of pregnant women and to make it a criminal offence for people to work if their immigration status forbids it.
Mr Johnson was condemned for stoking up fears about Turkish immigration during the referendum campaign - something he has denied.
...and the EU
The prominent Brexiteer repeatedly voted against greater EU integration before the referendum, and empowered Ms May to trigger article 50 in 2017.
He opposed Ms May's Brexit deal several times, resigning from the cabinet in protest in 2018.
However he supported her third attempt to get the deal through the Commons, which was still overwhelmingly rejected by MPs.