He is not just the youngest Prime Minister for 198 years, he is also the first ever to stand on the threshold of 10 Downing Street with a visibly pregnant wife at his side.
David Cameron has risen to the toughest job in politics with astounding speed, but at the last minute the summons to the Palace came at a moment he might not have chosen.
The news that Gordon Brown had accepted the inevitable was sprung on Mr Cameron, as it was on the nation. In a matter of minutes, the Conservative leader was no longer waiting in his office to hear from his negotiating team, who were shut away in Whitehall with their counterparts from the Liberal Democrats. Instead he was being driven in a silver limousine to his long-awaited audience with the Queen, with Samantha Cameron at his side.
Since Mr Cameron was still Leader of the Opposition he did not have the police outriders who will precede every time he is driven anywhere from now on. For one comical moment on the journey, he was held up behind a learner driver. He and Samantha arrived at the King's Door at Buckingham Palace about half an hour after Mr Brown's resignation, and were ushered up the larger staircase to the Queen's private apartments, in a room she uses for audiences with diplomats and foreign heads of state.
He was the 12th Prime Minister to present himself to the Queen in her long reign. After a short conversation, they shook hands for a photographer, and Mr Cameron began the slow drive back to Downing Street, where a small crowd had gathered at the gate to cheer him in. It was a downbeat affair compared with Tony and Cherie Blair's triumphal walk to the famous black door in 1997, because the abruptness of the changeover had not given the Conservatives time to organise themselves.
But Mr Cameron was unfazed as he stood at the microphone to give a well-crafted speech, paying tribute to the defeated Labour government, and promising a "proper and full coalition" in its place.
His first task , once he was through the famous door, was to put his Government together. Certain appointments were simple – George Osborne was confirmed as Chancellor, William Hague as Foreign Secretary, and Andrew Lansley as Health Secretary, the jobs they were promised when Mr Cameron was expecting the Conservatives to be in office on their own.
But Michael Gove's offer to surrender his prospective post as Schools Secretary to his Liberal Democrat opposite number, David Laws, was taken up, and Vince Cable was spotted going into the Treasury, an indication that he will be working alongside Mr Osborne, as Chief Secretary to the Treasury.
The Treasury will be the scene of some hard decisions. The Conservatives have promised to bring down the government deficit swiftly. The task was never going to be painless, but is made no easier now that they have coalition partners who are more protective of public services and who have demanded that people on annual incomes of £10,000 or less should be taken out of tax altogether, reducing the treasury's income. High up on Mr Cameron's agenda now is the Bill he has promised his new partners that will enable the Government to call a referendum on whether to change the system under which MPs are elected. The Liberal Democrats have not been granted the proportional representation that they wanted, but they have a chance of getting the alternative vote, a milder reform of voting.
Mr Cameron will also want an almost immediate meeting of the national security council so that he can be briefed on the situation in Afghanistan, and the new Home Secretary will have the relatively simple job of putting an end to the proposal to introduce ID cards .
As soon as David Laws walks into his new office, as Schools Secretary, he will need to address himself to the promised "pupil premium" which will direct money to state schools that take in pupils from disadvantaged backgrounds.
Danny Alexander, Nick Clegg's clever and quietly spoken chief of staff, was made Secretary of State for Scotland – a logical appointment when the Conservatives have just one MP north of the border.
These appointments come at a price. The day before yesterday, Mr Cameron had a shadow cabinet of 23 men and women who were looking forward to sitting in a Conservative Cabinet, and many more shadow ministers who also expected to be moving into government offices as soon as David Cameron was ensconced in Downing Street.
Today there are some disappointed people consigned to the Tory backbenches, who could yet become a source of considerable trouble for the young Prime Minister.
George Osborne, 38
He becomes the youngest Chancellor for more than a century. He has been David Cameron's closest ally in the drive to modernise the Tory party and managed his successful leadership campaign.
William Hague, 49
The former Tory leader (1997-2001) was brought back to the Tory front bench by David Cameron. The new Foreign Secretary, who also served in Sir John Major's Cabinet, is a strong Eurosceptic.
Andrew Lansley, 53
The new Health Secretary has known David Cameron since they worked together in Conservative HQ in the early 1990s. Has been an MP since 1997 and was made Tory health spokesman six years ago.
Vince Cable, 67
The deputy Liberal Democrat leader is expected to become Chief Secretary to the Treasury. A former economist and Labour adviser, he is seen as among the country's most popular politicians.
David Laws, 44
After a glittering City career, Mr Laws, who is likely to become Education Secretary, succeeded Paddy Ashdown as Liberal Democrat MP for Yeovil. He is regarded as on the party's modernising wing.
Nick Clegg, 43
The new Deputy Prime Minister has only been an MP for five years. He was previously a European Commission official and a Euro-MP. He succeeded Sir Menzies Campbell as leader of the Liberal Democrats in December 2007.
The day's events
07.39 Tessa Jowell says Lib Dems have invited Labour to talk.
08.20 Uncertainty prompts FTSE 100 to slump 1 per cent within minutes of markets opening.
08.27 George Osborne describes offer of a referendum on the Alternative Vote system as "final".
08.32 David Blunkett says a Lib-Lab coalition would spell electoral disaster for Labour and accuses Lib Dems of acting like "every harlot in history".
08.45 David Cameron leaves home warning Lib Dems it is "decision time".
09.15 Nick Clegg says he's as impatient as anyone for resolution to talks.
10.01 Peter Mandelson, Ed Balls, Lord Adonis, Harriet Harman and Ed Miliband leave Downing Street for Commons to lead talks with Lib Dems.
11.00 Alan Johnson arrives at Downing Street. He leaves 20 minutes later.
12.28 Gordon Brown leaves Downing Street for the Commons.
12.49 Ed Miliband describes the morning's talks as "good".
13.42 Lib Dems reveal Cameron and Clegg met for an hour in the morning.
14.00 Lib Dems and Tories resume their negotiations.
14.21 Brown returns to Downing Street.
14.30 John Prescott urges Lib-Lab pact.
14.55 Labour lead negotiators return to Downing Street.
15.32 Cameron says he's "in the dark" about the prospect of a deal.
15.45 Lord Falconer tells Brown to "call it quits now".
15.57 Andy Burnham says he is opposed to any Lib-Lab pact.
16.15 Former Labour chairman Ian McCartney says party should go home and prepare for opposition.
16.45 Tory MPs put on stand-by for a meeting during the evening.
17.40 Simon Hughes says he could accept Lib Dems joining with Tories.
19.07 Downing Street staff place lectern outside No 10.
19.18 Brown announces he is resigning as PM with immediate effect.
19.22 The Browns leave Downing Street for Buckingham Palace. Four minutes later they arrive, where he is addressed as "Prime Minister" for the last time.
19.32 Liberal Democrat negotiators leave talks to report back to Clegg.
19.35 Tory team emerges from talks to report back to Cameron.
19.43 Gordon Brown and his wife Sarah leave the palace.
19.46 Alistair Darling and his wife Margaret leave No 11.
19.49 Brown arrives at Labour Party HQ.
20.07 David Cameron arrives at Buckingham Palace with his wife.
20.34 Cameron leaves Buckingham Palace as Prime Minister.
20.41 Cameron reaches Downing Street.