They have both held one of the highest offices in the country under Johnson's premiership, and will spend the summer campaigning around the country to appeal to Conservative Party members for their votes before the winner is announced on 5 September.
But who are they and what do they stand for? Yahoo News UK summarises their lives and policies below.
Sunak is 42 and Truss will be 47 later this month. If Sunak becomes PM he would be the youngest leader of the UK in the post-war era and the 6th youngest since the creation of the office.
Sunak was educated at the prestigious Winchester College, a private school, where he was head boy. He went on to study Philosophy, Politics and Economics at Oxford and gained a Master's from Stanford University. His parents, Yashvir and Usha Sunak, were born in Kenya and Tanzania respectively, and grew up in India.
Sunak, a practising Hindu, got married in 2009 to Akshata Murty, who is the daughter of an Indian tech billionaire. The couple are estimated to be worth around £730 million, making them one of the wealthiest families in the UK, and have two young daughters.
Truss was initially raised in Scotland before moving to Leeds where she attended a state school.
She studied Philosophy, Politics and Economics at Oxford and is married to accountant Hugh O'Leary and the couple have two daughters.
Truss is still the foreign secretary after remaining loyal to Boris Johnson when he suffered a wave of resignations that toppled his government.
Sunak was Chancellor until he quit on 5 July, a resignation that helped trigger the collapse of the government.
He is now a backbench MP.
What are their plans to deal with...
1. The cost of living crisis
Both candidates have said that soaring inflation and the subsequent cost-of-living crisis is the most pressing matter facing the country - but they have offered very different solutions.
Sunak has said he would prioritise responsible management of the economy, which he says means rising inflation should be tackled before launching any tax cuts.
As Chancellor he announced £37 billion worth of funding to help people with the rising cost of living, mostly to help tackle rising energy bills.
He has said he will keep the 1.25 percentage point increase to National Insurance Contributions that came in to force in April 2022 as part of a boost to health and social care funding.
Sunak is also sticking to plans to hike corporation tax to 25% next year.
In many ways, Sunak's offer to voters is that if they liked his fiscal approach during COVID and the cost of living crisis as chancellor, then they know what to expect from him as prime minister.
Truss, on the other hand, has offered an alternate vision that is focussed on cutting tax much sooner. She has said she would cancel the increases in National Insurance and corporation tax as soon as she enters office.
She has also said she would scrap green levies on energy bills for two years to reduce their cost - they currently make up just under 10% of a household's annual energy bills.
Separately she has also said she wants to expand the marriage tax break which would allow couples to transfer their full £12,570 tax allowance to their partner.
To date, however, Truss has provided no outline of how these tax changes would be paid for. She wants to extend the length of time the government repays the enormous pile of debt it accrued during COVID.
Both Truss and Sunak have said they support the government's controversial scheme to deport illegal immigrants to Rwanda.
Truss has said she may ask Turkey to also become a deportation destination, but this has been ridiculed by the Turkish government.
3. Climate change
Both candidates have said they want to keep the commitment for the UK to reach net zero by 2050, but their statements do differ slightly.
Sunak has said he would keep the ban on new onshore wind farms but wants to introduce a target to make the UK energy self-sufficient by 2045 with a massive increase in offshore turbines.
Truss has also said she wants to keep the net zero target (in a way that "doesn’t harm businesses or consumers") but has also said she wants to temporarily cut green levies on energy bills.
She has also indicated she would review the ban on fracking.
Sunak has said he wants to be tougher on criminals, with extended sentences for those who refuse to turn up to their trials.
He also said he would give the justice secretary the power to personally overrule the parole board when dangerous defenders are up for release. This would be a controversial move and could potentially be seen as politicising a supposedly independent judiciary.
Sunak has also criticised the police saying the grooming gangs in Rochdale should "never be repeated" and said officers should be focused on "fighting actual crime" rather than "bad jokes on Twitter."
Truss has so far not mentioned any specific reforms to policing or the criminal justice system. She has previously described custodial sentences as being too long, that the "wrong people are in prison" and that prisons are too overcrowded to work.
Health and social care
Despite consistently being high up on the list of issues voters care most about, the NHS - and social care -has so far not received much attention in the leadership campaign.
The only significant mention by Sunak was during the leadership debates, where he said the NHS would continue to be his number one priority if he was elected as leader.
Truss has said she supports the extra money provided to the NHS to help tackle the backlog, but not the way it is being funded by the increase to National Insurance Contributions.
Sunak supported leaving the EU and has pledged to scrap or reform all remaining EU laws that are still in effect in the UK.
He has also said he wants to reduce red tape on the City of London.
Truss backed Remain during the referendum and has faced questions from Conservatives over her stance as well as her former membership of the Liberal Democrats.
She has recently said she was "wrong" to back Remain in the referendum and pledged to make Brexit work.