After more than two weeks of intense wrangling and with just days to go until crucial vote on the Queen's Speech, the Tories and the Northern Ireland party put the stamp on a deal that shores up the PM's majority in the Commons.
The arrangement dramatically boosts Mrs May's chances of clinging on as PM after the disastrous election that stripped her of her overall Commons majority.
But it has come at a significant price - with around £1billion of extra funding on schools, hospitals and infrastructure for the province over the next two years.
The DUP is also claiming credit for the Tories ditching two key manifesto pledges - which would have seen the end of the triple lock on pensions and curbs to winter fuel payments for pensioners. The deal will run until the next election - and be reviewed at the start of every parliamentary session.
Labour leader Jeremy Corbyn condemned the deal as a stitch up to help Mrs May 'cling to power'.
Nicola Sturgeon said it was 'grubby' and 'unfair' on Scotland.
The Tories have scrambling to secure a 'confidence and supply' arrangement since voters dealt Mrs May a bloody nose on June 8.
The smaller party will remain outside Government but ensure the Tories stay in office by supporting them on financial measures and no-confidence votes.
It means that Mrs May now has an effective majority of 13 in the House of Commons, once the 10 DUP MPs are added to the 317 Tories.
After initial signs of progress, the Conservatives were embarrassingly forced to row back on an announcement that agreement had been reached.
Talks dragged on for more than a fortnight before finally reaching a conclusion this morning.
Mrs May and DUP leader Arlene Foster smiled as their respective chief whips, Gavin Williamson and Jeffrey Donaldson, signed the document in No10 this morning.
Mrs Foster said she was 'delighted' an arrangement had been agreed.
The Prime Minister said the DUP and the Tories 'share many values' and the agreement was 'a very good one'.
Speaking in No 10, Mrs May said that parties 'share many values in terms of wanting to see prosperity across the UK, the value of the union, the important bond between the different parts of the United Kingdom'.
'The agreement we have come to is a very, very good one, and look forward to working with you,' she told the DUP politicians.
Speaking in Downing Street after signing the deal, Mrs Foster said the agreement would deliver 'stable government in the UK's national interest at this vital time'.
The DUP leader said she had secured a £1billion investment for the province over two years plus new flexibility on £500million already committed.
That is equivalent to an increase of around 4 per cent on the current annual budget for Northern Ireland.
The Tories have given up manifesto pledges to axe the triple lock on pensions and cut winter fuel allowance payments to wealthy pensioners.
The Armed Forces covenant will be extended to Northern Ireland and the two parties have recommitted to spending 2 per cent of GDP on defence.
Mrs Foster said: 'Throughout these discussions our guiding principle has been our commitment to acting in the national interest, in accordance with our shared objective of strengthening and enhancing our precious union.
'In concluding this agreement, we have done so to secure our nation, building the prosperity of all and supporting our exit from the European Union to benefit all parts of the United Kingdom.'
She added: 'Our aim in this negotiation has been to deliver for all of the people of Northern Ireland
'The measures we are announcing will be to benefit all of our people.
'They will boost the economy, invest in new infrastructure as well as investing in the future of our health and education sectors and a range of other measures.'