Former GOP presidential candidate Ben Carson is now President Trump's housing and urban development secretary. Here's what you need to know about the neurosurgeon. (Sarah Parnass,Osman Malik/The Washington Post)

President-elect Donald Trump has nominated retired neurosurgeon Ben Carson to run the Department of Housing and Urban Development, an unconventional choice that underscores Trump’s willingness to forgo traditional policy expertise in some Cabinet positions to surround himself with allies.

Carson — who ran against Trump in the wide field of Republican presidential primary candidates and has never held political office — is the president-elect’s highest-profile African American supporter and confidant. A conservative political celebrity, the 65-year-old Carson recently cast doubt on his suitability for a Cabinet role, saying he would be “like a fish out of water” as a federal bureaucrat.

“I am thrilled to nominate Dr. Ben Carson as our next Secretary of the U.S. Department of Housing and Urban Development,” Trump is quoted as saying in a statement released by his transition team Monday. “Ben Carson has a brilliant mind and is passionate about strengthening communities and families within those communities.”

A review of some of Carson’s political commentary on housing policy reveals his views to be at odds with some of the anti-segregation policies championed by minority groups, including the NAACP. On at least one occasion, he criticized efforts by the Obama administration to use its regulatory powers to improve racial integration in housing.

If confirmed by the Senate, Carson will be enlisted to serve as Trump’s unofficial envoy to African Americans and to lead the charge to repair what Trump has described as America’s blighted and dangerous inner cities, a dystopian, racially charged image criticized as frequently at odds with reality. He will also lead Trump’s efforts in an arena where Trump and members of his family have been accused of flouting the law in their own business ventures.

Trump is also expected on Monday to formally name retired Marine Gen. James N. Mattis as his defense secretary. Trump shared his intentions Thursday during a raucous “thank you” rally in Cincinnati.

Carson’s lack of experience in the housing sphere raised concerns among advocates for vulnerable Americans.

“With many qualified Republicans to choose from with deep knowledge of, and commitment to, affordable housing solutions for the poorest families, and with the housing crisis reaching new heights across the country, Dr. Carson’s nomination to serve as HUD Secretary is surprising and concerning,” Diane Yentel, president of the National Low Income Housing Coalition, said in a statement.

Trump’s HUD pick follows his decision to name South Carolina Gov. Nikki Haley (R), the daughter of Indian immigrants, as ambassador to the United Nations and former labor secretary Elaine L. Chao as his choice for transportation secretary. Chao in 2001 became the first Asian American woman to be named to a Cabinet post and went on to head the Labor Department under President George W. Bush for eight years. These two appointments, along with Carson’s and Trump’s selection of Betsy DeVos as his choice for education secretary, confirm that the president-elect is seeking a measure of racial and gender diversity in his emerging administration.

Carson and the president-elect were not always as friendly as they are now.

In November 2015, as the Republican presidential primary cycle was heating up, Trump used Carson’s description of himself as a violent adolescent to attack his rival at campaign rallies.

Noting Carson’s admissions in his 1996 autobiography, interviews and public statements that, growing up, he had tried to attack his mother with a hammer and almost stabbed a friend, Trump said Carson had a “pathological temper” that was incurable, comparing it to “child molesting.”

“If you’re a child molester, a sick puppy, a child molester, there’s no cure for that,” Trump said in Fort Dodge, Iowa. “There’s only one cure, and we don’t want to talk about that cure. That’s the ultimate cure. No, there’s two — there’s death and the other thing. But if you’re a child molester, there’s no cure, they can’t stop you. Pathological, there’s no cure.”

Carson was also critical of Trump at times, but without the same level of intensity. He said in March that he might have “preferred” a different Republican presidential nominee but that “we’re only looking at four years” if Trump fails as president.

A Seventh-day Adventist, Carson entered the political sphere after admonishing the audience at the 2013 National Prayer Breakfast — including President Obama.

Standing a short distance from Obama, he said the United States faced “moral decay” and “fiscal irresponsibility” similar to the end of the Roman Empire. Soon, he was making the rounds as a commentator in print and on Fox News, where he was often featured as a voice critical of the Affordable Care Act.

Carson does not have specific experience with the activities of HUD, which manages low-income housing assistance, facilitates financing for homeownership and administers fair-housing programs. But he has a documented position on fair-housing policies that has advocates nervous about what protections a Trump administration might dismantle — particularly for racial desegregation.

Last year, then-candidate Carson forcefully criticized an Obama administration rule requiring cities and towns to publicly report racial bias in their housing patterns, saying it would “fundamentally change” communities by requiring affordable housing to be built in wealthier neighborhoods.

“These government-engineered attempts to legislate racial equality create consequences that often make matters worse,” Carson wrote in the Washington Times. “There are reasonable ways to use housing policy to enhance the opportunities available to lower-income citizens, but based on the history of failed socialist experiments in this country, entrusting the government to get it right can prove downright dangerous.”

Trump and members of his family have come under scrutiny for allegedly flouting fair-housing laws in their real estate business. In 1973, the Justice Department sued Trump and his father, Fred Trump, for allegedly discriminating against black apartment applicants. Trump filed a counterclaim and, after nearly two years of fighting, struck a deal with the government. The Justice Department declared it a victory for the enforcement of the Fair Housing Act. Donald Trump also declared victory, in part because the agreement stated that Trump made the deal without acknowledging he was at fault.

Throughout the 2016 campaign, Trump drew criticism for painting urban communities as hellishly violent and benighted.

“You have so many things, so many problems, so many horrible, horrible problems,” Trump told a rally before the election. “The violence. The death. The lack of education. No jobs. We’re going to work with the African American community, and we’re going to solve the problem of the inner city.”

If confirmed, Carson will join Trump’s Cabinet with a devoted following of conservative Christians, tea party supporters and fans of his achievements in pediatric neurosurgery, which were portrayed in the made-for-TV film “Gifted Hands.”

“We have much work to do in strengthening every aspect of our nation and ensuring that both our physical infrastructure and our spiritual infrastructure is solid,” Carson wrote Wednesday on Facebook.

He also brings a lengthy history of gaffes and odd, controversial statements unusual for an incoming Cabinet secretary: He has called Obama a “psychopath,” compared the Affordable Care Act to slavery and said the contemporary United States is akin to Nazi Germany. In one instance, he said gun control made it possible for the Holocaust to occur.

Carson’s personal history gives him a pronounced, often controversial view on the roots of urban poverty.

[Carson’s positions on poverty create tension with rags-to-riches life story]

Born into a struggling family on the southwest side of Detroit, Carson was educated at Yale University and the University of Michigan before he began a celebrated surgical career at Johns Hopkins University in Baltimore. He credits his achievements to his Christian faith and the high expectations of his mother, an illiterate domestic servant who sought never to take government assistance, though she sometimes did.

“She didn’t like the idea of dependency,” Carson told The Washington Post last year. “Even if she sometimes took government aid, she always wanted to be independent. She would get in arguments with others who would say, ‘There’s aid for dependent children — you don’t need to be working.’ ”

Carson’s social and scientific views are also far to the right. According to past statements, including on the 2016 campaign trail, he views being gay as a choice, doubts human activity contributes to global warming and thinks abortion should be outlawed in all circumstances, including in cases of rape and incest.

John Wagner contributed to this report.