"If he's got good people around him, he'll be fine," said 67-year-old backer Sue Busby, a grassroots member of the governing Conservative Party that looks set to propel Johnson to power this week.
The former London mayor has wooed Conservatives with a promise to get Britain out of the European Union and if elected party leader, he will take over from Theresa May as prime minister on Wednesday.
But critics are alarmed at how Johnson seems to struggle with the finer points of Brexit, and question his attention to detail in previous jobs.
As foreign minister, he misrepresented the case of a British-Iranian woman held in Tehran, and as London mayor, was accused of backing big projects that turned out to be expensive flops.
However, some of those who worked with him while he ran London from 2008 to 2016 say he surrounds himself with talented people.
"Although he's very much in control, and it's his vision, he is good at delegating," said Victoria Borwick, who served for three years as deputy mayor.
Another former colleague, who asked to remain anonymous, said Johnson was "very much 'big vision'.
"Once he got a team he trusted, he'd leave you to get on with it," he said.
While he could master the detail when required, on some issues, "there's an element of winging it".
Lots of views
Johnson has brought back some of the old London team to help him in his leadership bid, including former chief of staff Edward Lister and former communications chief Will Walden.
The campaign also includes many current and former MPs and ministers, some of them long-time supporters and others who have fallen in behind the man who seemed destined from the start to win.
But as a result, he is leading a coalition of people who do not always agree.
Johnson has promised to take Britain out of the EU with or without a deal on October 31, and says all his ministers must back this approach.
But his campaign supporters range from hardline eurosceptic MP Jacob Rees-Mogg to Health Secretary Matt Hancock, who previously said he did not believe "no deal" was a viable policy.
"A lot of those people will be looking for rewards in terms of jobs in government" if he wins, noted Catherine Haddon of the Institute for Government think-tank.
"There are lots of views about what Theresa May did wrong and how things should be done differently.
"How much will he have to intervene to sort out any disputes between them?"
Another important issue is to what extent ministers will themselves have to manage the unpredictable and gaffe-prone Johnson if he reaches Downing Street.
Despite starting the race with a huge lead, Johnson's campaign team has played it safe, limiting his media appearances and toning down his rhetoric.
When he was mayor, his staff also sought to keep him away from certain delicate situations, according to London's Evening Standard newspaper.
"He never once met the rail unions directly as mayor, for instance... They kept him away because they knew it would go wrong," it reported.
Johnson is proud of his record in London, pointing to low levels of crime, investment in transport and housing as proof of his ability to get things done.
But critics cite expensive projects such as a cable car across River Thames, an aborted garden bridge and his decision to buy second-hand water cannons that police were not permitted to use.
One supporter told AFP the cable car was underused because it was in the wrong place, which they insisted was not Johnson's fault.
But, for his critics, this over-reliance on his team goes to the heart of the problem.
"He's great on rhetoric but lousy on delivery," Steve Norris, a former Conservative candidate for London mayor told The Guardian newspaper.