True, Johnson had once quipped that he was more likely to be reincarnated as an olive than he was to become Prime Minister, but that was just a time-buying joke. His colleagues had no doubt of his ambition to seize the top job in politics.
Many were convinced that the former London Mayor -- a liberal, progressive cosmopolitan -- had only lined up with his party's right wing and become a late joiner of the Leave Europe camp in order to help undermine David Cameron and further his own leadership ambitions.
In the Leave camp, Johnson joined another of David Cameron's former friends and allies, Justice Minister -- and now candidate for the Conservative Party leadership -- Michael Gove. But if you subscribe to the common conspiracy theory, it was then that it began to go badly wrong.
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Gove's Daily Mail journalist wife Sarah Vine confessed in print that she had told her husband, 'you were only supposed to blow the bloody doors off,' using the analogy of the famous bullion robbery movie 'The Italian Job.' Instead Gove and Johnson, combining as the spearhead of the official Leave campaign, scuppered the Prime Minister and a lot more.
With an anti-politics mood engulfing the country and anti-immigrant sentiment running high thanks to the separate efforts of the UK Independence Party (UKIP) led by Nigel Farage, the Leave camp won the referendum as much to the amazement of Gove and Johnson as it was to stock markets and opinion polls.
It soon became apparent that Brexit campaigners simply had no plan for that eventuality. And while worldwide markets lost $3 trillion in the two trading days after the vote, the airwaves were dominated by a triumphalist Farage and an affronted Scottish First Minister Nicola Sturgeon, whose citizens had voted to stay in the EU.
Scotland likely to seek independence after EU vote, First Minister says
Gove and Johnson went to ground. Johnson emerged only to play in a cricket match and pen an article for British newspaper The Telegraph in which he blandly assured readers that there was no threat to the pound. Since it was important for Britain to stay in the Single European Market, he also said, we might have to continue with the EU's Free Movement of People.
Crowd pleaser v. statesman
Tory MPs who had assured voters that tough action on immigrant numbers would follow a Leave vote were appalled by his carelessness. If Cameron's penchant for last minute initiatives to squeeze out of trouble had him dubbed 'The Essay Crisis Prime Minister,' Johnson was suddenly exposed as a man with a ready and entertaining article about everything and no serious policy about anything; a man with a golden pen and little sense of responsibility.
His colleagues know his value as an entertainer and crowd-pleaser but the new situation required a statesman with a vision. Johnson simply didn't measure up.
An email from Vine to her husband Gove, revealed in the UK press, warning him to have witnesses with him and get promises in writing from Johnson in their exchanges about a leadership campaign, revealed distrust between the two major architects of the Leave campaign.
The email also worried Conservatives because it revealed that the editor of British newspaper Daily Mail, Paul Dacre, and the Times and Sun proprietor Rupert Murdoch instinctively disliked Johnson. They would only support him, said Vine, if he and Gove were in tandem.
Government is not a game
Johnson's weak spot was highlighted on Thursday morning when Home Secretary Theresa May opened her leadership campaign with a clear list of priorities and a not-very-coded warning: 'Some need to be told that what the Government does isn't a game. It's a serious business that has real consequences for people's lives.'
May presented herself as a state-educated realist who understood how hard life could be for ordinary folk. Two other candidates -- Stephen Crabb and Liam Fox -- emphasized their state education and comparatively humble origins in contrast to the gilded backgrounds of Cameron and Chancellor George Osborne.
For the Eton-educated Johnson, even that was becoming a liability in the new class-conscious political climate with all the focus on 'Tory toffs.' By then, support was ebbing away from Johnson and his game was up. At that stage, only a Winston Churchillian roar could have brought him back into the game. But he did not have it in him.
Gove, clearly having failed to extract from Johnson the assurances he had sought, jumped ship to run his own bid for the leadership. Johnson, having announced that he wasn't the man to lead the party then went off -- presumably to pen his next article.
Britain's Brexit vote: Full coverage
Editor's Note: Robin Oakley was political editor and columnist for The Times newspaper in London from 1986 to 1992, the BBC's political editor from 1992 to 2000, and CNN's European Political Editor between 2000 and 2008. The opinions expressed in this commentary are solely his.