Poor Boris. He only agreed not to be himself for a couple of months. Now London's new mayor must spend two years as a closely guarded hostage of the Cameron makeover squad, only getting time off for good behaviour - to chair the Metropolitan Police Authority. Certainly, glittering prizes lie ahead: City Hall, a second term, the Cameron cabinet, ultimately No 10 itself. But what shall it profit a man if he gain the whole world and lose his own soul?
Johnson's best quip of the weekend - "I was elected as new Boris and I will govern as new Boris" - was apt after an acceptance speech from the new Labour textbook (albeit somewhat bold in acknowledging "huge and growing divisions between rich and poor"). His grace in victory - and Ken Livingstone's magnanimity in defeat - offered a moving tableau of democratic transition far more elevated than the dispiriting campaign that preceded it.
For the real Livingstone had gone missing, too, turning into any other incumbent: running far too heavily on a mostly creditable record, but with no memorable pledge for London's future. One cabinet minister told me: "You would expect, eight years in, Ken to be running on his personality to hide his record; instead, he is running on his record to hide his personality."
Easily the best-informed candidate, Livingstone so often ended up contesting crime and transport statistics, rather than making a political argument. Red Ken was finally new Labour at last and seemed resigned to his fate.
Pretending to mount a serious contest in the mayoral race cost the Liberal Democrats two Assembly seats. The third party almost always performs terribly in elections held under proportional representation. This party of grown-up pluralism was the only one - unlike the Greens, the Left List and even the British National Party - which could not work out how to play the mayoral second preferences game.
After the polls closed, Brian Paddick revealed that his own second vote had gone to Lindsey German of the Left List. Might Nick Clegg's Tory-leaning image be mere cover for a secret pact between the Lib Dems and the Socialist Workers Party?
With all eyes now on the Boris show, Johnson's critics are already repeating the mistake they made during the campaign by gleefully anticipating a gaffe-filled mayoralty that will wreck David Cameron's project. Much of media-age politics is to do with managing expectations. Johnson benefits as much as Ronald Reagan or George W Bush ever did from being seriously "misunderestimated". Which other candidate would have got away with floundering and being roughly £100m out on their sums for buses in the televised mayoral debates?
As with Bush against Al Gore in the 2000 US election, Johnson's hustling for the levers of power reminds us of the apophthegm of his 18th-century namesake: "It is not done well; but you are surprised to find it done at all."
In office, today's Johnson may struggle to meet the basic competence threshold. His friend Charles Moore spoke affectionately of the impossibility of getting him to file his Telegraph column ("Quite a lot of journalists are late . . . Boris was stratospherically late").
If he merely remembers to put his trousers on every morning and get to work, Johnson's mayoralty will be acclaimed as a triumph. But the real test must be the same any other mayor would face: delivery.
That - with Johnson presented as a hands-off "chairman of the board" - is truly a test of the Cameron project. A friendly media stands ready to acclaim his unexpected ease during his first hundred days in office, as the former think-tank head Nick Boles choreographs a flying start.
But, after that, how often does his stated "policy" amount to anything more than an exhortation to "social responsibility"? And, having made campaign capital out of knife crime and promising a turnaround, the next stabbing somewhere in the streets of south London will be on his watch.
Whatever the nervousness about Boris Johnson, there is still cause for Cameroonian celebration. His victory defuses the "Eton/ Bullingdon toff" attack. The right claims this charge is an illegitimate left-wing retreat to the "politics of envy". In fact, it is the Tory party which has come full circle. For 40 long years after Sir Alec Douglas-Home lost to Harold Wilson, the Conservative Party insisted that its candidates for the premiership - Edward Heath, Margaret Thatcher, John Major, Michael Howard - should have been to state schools. Douglas Hurd couldn't break that mould in 1990.
Now, Johnson has made it possible for Old Etonians to hold power again. Background should not matter. In our society of stalled social mobility, there should be no glass ceiling on privilege at the very top. But Labour cannot decide how to attack. Are Johnson and Cameron empty PR salesman, or unreconstructed ideologues of the hard right?
The truth is that they are conservatives. They will accept what has been done, but may not want to go any further. They will wear their core beliefs very lightly if that is the price of winning power. What, if anything, they want to do with power, we don't know. But we can now watch Boris Johnson to find out.
Sunder Katwala is general secretary of the Fabian Society