The British foreign secretary was accused of "backseat driving" by a fellow minister and rebuked by the statistics watchdog for his article outlining how "glorious" life will be outside the European Union.
But his 4,000-word essay also drew praise from eurosceptic members of the ruling Conservative party for his demand for a clean break with the EU, in particular on the fraught issue of financial payments.
"You could call it backseat driving," Home Secretary Amber Rudd, who campaigned to stay in the EU, told BBC television on Sunday.
But May herself insisted that she was in control, telling reporters on a trip to Canada: "The UK government is driven from the front and we all have the same destination in our sights."
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May's grip on power remains fragile after losing her parliamentary majority in the June election, and with it her ability to force through Brexit.
But May has dismissed talk of a challenge from her foreign minister. "Boris is Boris," she said earlier on Monday, according to British media.
Publicly, he has pledged his support for May, who he will join at the United Nations in New York this week.
"What I'm trying to do is sketch out what I think is the incredibly exciting landscape of the destination ahead," Johnson told the BBC after arriving in New York.
"People want to know where we're going," he added, describing his lengthy Brexit article as "an opening drum roll" to May's speech in Florence.
With questionable timing -- on Friday night, hours after a bomb attack in London -- he laid out the opportunities of Brexit with his usual flair in an article for the Daily Telegraph.
"I am here to tell you that this country will succeed in our new national enterprise, and will succeed mightily," he wrote.
Johnson argued against paying for continued access to Europe's single market -- a possibility other ministers have left open.
The head of the independent UK Statistics Authority, David Norgrove, rebuked Johnson for a "clear misuse of official statistics", saying the figure confused gross and net contributions.
But Eurosceptic Conservative lawmakers welcomed Johnson's intervention, with one, Jacob Rees-Mogg, saying he was "loyally putting forward government policy... With panache".
Another, Nadine Dorries, wrote on Twitter: "The PM is not furious. It was a drum roll for her speech next week. It punctured the gloom.