Then the hard work begins.
Burnham, 56, faces a long list of problems, from Britain's underperforming utilities to anaemic economic growth, and must first unveil his top team of ministers - already the subject of much debate in the governing Labour Party.
BURNHAM TO LAY OUT NEW GOVERNMENT PATH
Describing his election to become Labour leader as "the most significant change moment in our politics for 40 years", Burnham has promised to radically change Britain's political system to quickly raise living standards.
"The government I lead will confidently lay that path out starting next week," he said on Friday when he promised to offer Britain the hope and change the country "was crying out for".
"It will take us to a country where life is more affordable and all people and places are lifted from where they are now."
That message was made directly to Labour lawmakers, who, after he returned to parliament last month, see him as one of the few politicians who can reduce the threat from veteran Brexit campaigner Nigel Farage's populist Reform UK party - something they doubted the unpopular Starmer could do.
Burnham's first hurdle will be the appointment of his cabinet team, particularly his choice of finance minister. Friction in this crucial partnership at the heart of government has led to the downfall of previous administrations.
An early frontrunner for the position, energy security and net zero minister Ed Miliband, has been on the receiving end of some hostile briefing and interior minister Shabana Mahmood now appears to be favourite for the job.
Burnham called on his party to ignore "speculation" on Friday and said he had not yet decided on his team.
His early policy decisions will also face scrutiny, with his team keen to move on with his agenda quickly.
On Sunday, one early move was to scrap plans for all employees to hold a digital identity document, a scheme designed to tackle illegal migration but deemed to be a "fiasco" by a cross-party committee of lawmakers.
More attention will be paid to Burnham's subsequent decisions, particularly on taxation and spending, oil and gas, and underperforming utilities companies, which he wants to see under stronger public control.
On Sunday, his ally, deputy Labour leader Lucy Powell suggested Thames Water, a debt-stricken utility, could be brought under "special measures", meaning it would operate under government control.
"He wants to refocus and reprioritise the government's resources, the government's attention onto his priorities of tackling the cost of living, really rewiring the way the country, the economy works," she told Sky News.
