David Buttress, a former head of the Just Eat online food delivery service, has been asked to develop new business-led ideas to help people struggling with falling living standards.
He was described as boasting a “wealth of experience” and ready to bring the “vigour and ingenuity of business” to the role, working with the Cabinet Office.
But, in January, as Mr Johnson was engulfed by the scandal over the lawbreaking No 10 parties, Mr Buttress tore into the prime minister’s failings on Twitter.
“Never confuse an expensive education with intelligence or integrity,” he tweeted.
“I don’t think Boris is particularly blessed with either, imagine if he had not had the many privileges he has experienced. Does anyone believe he would be PM on merit? Not a chance, mate.”
Later that month, Mr Buttress also posted: “Why is it that the worse [sic] people often rise to the highest office and stay there!?”
And he added: “Boris has to go, he just has to. You can’t survive judgment like this.”
The prime minister’s spokesman declined to say if ministers were aware of the new tsar’s past criticisms, but insisted “due diligence” had been carried out before the appointment.
Mr Buttress is also a supporter of Welsh independence, once linking child poverty to being part of the United Kingdom.