America had never seen a first lady like this before.

Now holding the post in her final days, Michelle Obama is set to leave behind an impressive legacy.

Widely seen as down-to-earth and charming, the Harvard-educated lawyer became a pop culture figure, and developed a number of passion projects during her time in the White House.

Growing up, Michelle LaVaughn Robinson was a good student from a working-class background.

"I come here as a daughter — raised on the south side of Chicago by a father who was a blue-collar city worker and a mother who stayed at home with my brother and me," she told the Democratic National Convention in 2008.

"And thanks to their faith and hard work, we both were able to go on to college, so I know first-hand from their lives and mine that the American dream endures."

From those humble roots, Mrs Obama went on to become a Princeton and Harvard-educated lawyer.

Barack Obama and new wife Michelle pose for a photo on their wedding day on October 18, 1992. ( Obama for America )

She met her husband more than a quarter of a century ago while working in Chicago, and she became his fierce champion on the world stage and each campaign trail.

She defended her husband's more controversial policies, including the Obamacare changes to health, and she helped to make him appear human.

"Millions of Americans who know that Barack understands their dreams, millions of Americans who know that Barack will fight for people like them, and that Barack will bring finally the change we need," she said.

"And in the end, after all that's happened these past 19 months, see the Barack Obama I know today is the same man I fell in love with 19 years ago."

'She focuses on fairness'

Over the years, Mrs Obama championed causes close to her heart, among them, racial equality and women's rights.

Her biographer Peter Slevin said Mrs Obama would be remembered for the work with girls, particularly those of colour who lack opportunity.

"She focuses on fairness, on dealing with this high rate of inequality — economic, racial, gendered, class inequality in this country. Her initiatives are focussed on that," he said.

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Passionate about improving the health of American children, Mrs Obama started a fitness campaign and created an organic kitchen garden at the White House, inviting children to plant and harvest with her each season.

Mr Slevin said it is a form of soft diplomacy through pop culture, using social media and popular programs like Ellen and Sesame Street to deliver serious messages.

"She has been able to target her messages to the audience she wants to reach," he said.

"She does it through social media, she does pretty much every social media that's existed and some that a lot of adults don't even know a lot about."

"She did (James Corden's) Carpool Karaoke a few weeks ago that has now been viewed 46 million times, and she used humour and she sang, but embedded in that video that has now been now watched so many times was a message about girls education."

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'Some of the criticism is racist'

That drive to empower women moved up a gear when Donald Trump became a presidential candidate.

Mrs Obama took to Hillary Clinton's campaign trail earlier in the year to call out Mr Trump's behaviour, in a speech that has been called "remarkable", "epic" and "powerful".

"Strong men, men who are truly role models, don't need to put down women to make themselves feel powerful," she told the crowd.

"People who are truly strong lift others up, people who are truly powerful bring others together."

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And she made headlines around the world again for one particular line in her 2016 Democratic National Convention speech.

"I wake up every morning in a house that was built by slaves and I watch my daughters — two beautiful, intelligent, black young women — playing with their dogs on the White House lawn.

"And because of Hillary Clinton, my daughters — and all our sons and daughters — now take for granted that a woman can be president of the United States."

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Mrs Obama has drawn criticism from the political right for her support of President Obama's divisive policies, like gay rights and increased gun control.

"From the right, from conservatives, Mrs Obama is seen as something of a the leader of a nanny state," Mr Slevin said.

"While much of the criticism of her is on political terms, you know, Republicans criticising her as a progressive Democrat, undoubtedly some of the criticism is racist in this country."

But Mrs Obama's strong support base has been unwavering.

"Among Democrats, she has a 95 per cent approval rating and in fact in a very polarised country, she has an approval rating among all adult Americans of about 60 per cent which is quite high."

President Michelle Obama?

Many people are wondering if we will ever see a President Michelle Obama in the future, but Mr Slevin believes it is very unlikely.

"She insists she has no interest whatsoever," he said.

"Politics is not Michelle Obama's world and she has said quite consistently that she will not run for office."

One thing is for sure, Mrs Obama is leaving big shoes to fill for the next first lady, or the first man.