Who is he?
Bannon was a key figure in steering Trump's campaign and his rise to the presidency.
The two men met in 2011 when Trump first expressed a desire to run for president. They immediately "hit it off," even as Trump's initial ambitions fizzled out.
In August 2016, Trump brought Bannon on board as his campaign manager just as the Republican candidate's election bid appeared to be faltering.
After winning the election, Trump has kept Bannon by his side.
What's his role in the Trump administration?
He is Trump's chief White House strategist and senior adviser.
The 63-year-old former businessman is also described as the 'mastermind' of Trump's radical policies and the string of executive orders, that included a temporary ban on the citizens of seven Muslim-majority countries and the admission of refugees into the US.
Trump has also given Bannon a permanent seat in the National Security Council (NSC), a body that advises the president on foreign policy and national security.
This decision raised concern because the position has traditionally been reserved for people like the secretary of the state or the director of national intelligence.
"It's a stunning thing that a white supremacist, Bannon, would be a permanent member of the National Security Council," House Minority Leader Nancy Pelosi said.
Is he a "racist manipulator" or a "brilliant tactician"?
He's never held public office but has become a highly influential figure among the far-right as the chairman of Breitbart News, which critics say promotes the idea that America needs to be reclaimed.
It is a theme Trump hammered during his presidential bid, adding more weight to the belief that Bannon was guiding the Republican candidate.
"Bannon was the main driver behind Breitbart becoming a white ethno-nationalist propaganda mill," said the Southern Poverty Law Center, an organisation that monitors hate groups across the US.
Breitbart News is accused of promoting Islamophobia, anti-semitism, sexism and racism but its supporters say Breitbart is just an anti-establishment platform.
But Trump aides have been quick to rubbish any suggestions that Bannon has any ideological influence on the US president.
"He has got a Harvard business degree. He's a naval officer. He has success in entertainment," another Trump adviser, Kellyanne Conway said, describing him as a "brilliant tactician."
Bannon has been labelled Islamophobic, anti-Semitic, misogynistic and populist. What has he said?
"We're now, I believe, at the beginning stages of a global war against Islamic fascism."
"He said that he doesn't like Jews and that he doesn't like the way they raise their kids to be ‘whiney brats' and that he didn't want the girls going to school with Jews." - Bannon's ex-wife said in a court disposition
"They're either a victim of race. They're victim of their sexual preference. They're a victim of gender. All about victimhood and the United States is the great oppressor, not the great liberator," he said in a 2011 interview
Critics point to articles published on Breitbart under Bannon as further proof.
Bannon's ex-wife also accused him of domestic violence in 1996.
A populist who just wants to give jobs back to Americans
"It's everything related to jobs. If we deliver, we'll get 60% of the white vote and 40% of the black and Hispanic vote, and we'll govern for 50 years," Bannon said in an interview.
Bannon has worked as an investment banker with Goldman Sachs who went to Harvard Business School. His supporters say he is intelligent enough to "deliver."
He is also the founder of the Government Accountability Institute, which says its mission is to expose government corruption.
Commentators point to praise by white nationalists as proof of Bannon's views on race.
"You have an individual, Mr. Bannon, who's basically creating the ideological aspects of where we're going," former Ku Klux Klan leader David Duke said in an interview after Trump picked Bannon as his chief political strategist
"Perhaps The Donald IS for 'REAL," the Chairman of the American Nazi Party, Rocky J. Suhayda, said of the development.
Does he really call the shots?
"Where Bannon is really having his instinct is on the policy front. Which policies? All of them. He's Trump's facilitator," a Trump ally told Time magazine.
Trump's outline for domestic and foreign policy have largely been inspired by Bannon, observers say.
Bannon regularly invited Trump onto his radio show at Breitbart during the 2016 campaign and "coached" him on key issues.
Trump's campaign promises of bringing jobs back to America, building walls, empowering ordinary people, pulling out of unfavourable international trade deals and keeping refugees out, are all rooted in what commentators term as "Bannonism."