New British Prime Minister Boris Johnson Wednesday sent a strong signal with a major personnel move, naming political strategist Dominic Cummings, best known for his polarizing role as the campaign director of the successful “Vote Leave” Brexit campaign, as one of his most senior advisers.

The outspoken Mr. Cummings is credited with helping mastermind the surprise victory in the national referendum on leaving the European Union, coining the movement’s slogan, “Take Back Control.” Mr. Johnson, who won a Conservative Party leadership race largely on the strength of his claims he will carry Brexit to a successful conclusion by the Oct. 31 deadline, is surrounding himself with several pro-Brexit loyalists, including Lee Cain, in line to be the new prime minister’s director of communications, and Rob Oxley, the new press secretary.

While guiding the “Vote Leave” movement, Mr. Cummings was known for his pugnacious and pointed criticisms of “Remainers.” On his personal blog, he once dismissed anti-Brexit Tory lawmakers as a “narcissist-delusional subset” and “useful idiots.”

This past March, he was found in contempt of Parliament for failing to appear for questioning surrounding reports of fake news reports during the 2016 Brexit debate. The Vote Leave movement’s relations with Cambridge Analytica, the British firm which harvested Facebook data to aid political campaigns, also drew scrutiny.

The appointment was hailed by Brexit partisans and quickly condemned by those who have tangled with Mr. Cummings in the past.

Sarah Wollaston, a former Conservative MP and now an independent, said Mr. Johnson showed “an appalling error of judgment,” while Layla Moran, an MP for the center-left Liberal Democrats, called the selection “dangerous.”

Mr. Cummings “has shown nothing but contempt for our judicial system, and parliament, indeed any vehicle that asks questions of him and his methods and see the eyes of scrutiny set upon him,” she wrote in The New Statesman.

Brexit was not Mr. Cummings’ first experience in the long domestic debate over relations with Europe. From 1999 to 2002, he ran a successful campaign to keep Britain from adopting the euro as its official currency.

Following the Brexit vote, Mr. Cummings worked for Michael Gove, a Conservative Party ally turned rival to Mr. Johnson.

He’s been called blunt and outspoken, quick-witted and disrespectful. Oscar-nominated actor Benedict Cumberbatch memorably portrayed him in a popular British miniseries on the Brexit campaign.

Mr. Cummings will reportedly share chief of staff duties with Sir Edward Lister, a key aide to Mr. Johnson from his time as mayor of London.

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