Instead, she turned inward.

“I didn’t want to be influenced by how they defined the role,” Mrs. Obama once said. She instinctively knew she had to define the job “very uniquely and specifically to me and who I was.”

That meant doing it her way: shaping the role around her family, specifically her two young daughters, and not letting her new responsibilities consume her.

Throughout her eight years, Mrs. Obama has been a powerful, if somewhat enigmatic, force in her husband’s White House. She chose her moments in the often unforgiving spotlight with great care and resisted pressure to become more engaged in the mudslinging of partisan politics.

At times, she’s been more traditional than some expected—or wanted from this first lady. At other times, she’s been eager to update stuffy conventions associated with the office.

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As she navigated her way through, the woman who grew up on the South Side of Chicago discovered a talent for television and a comfort with Hollywood A-listers, haute couture and social media. And she used all of those elements to promote her causes — childhood obesity, support for military families, girls’ education — with at least some success.

When she leaves the White House next month just a few days after celebrating her 53rd birthday, Mrs. Obama will do so not just as a political figure, but as a luminary with international influence.

Friends say she charted that path largely on her own.

“What she did was she sort of listened to herself and allowed her own inner voice and strength and direction to lead her in the way that felt most authentic to her,” Oprah Winfrey told The Associated Press. “And I think watching somebody makes you want to do that for yourself.”

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Mrs. Obama grappled with the childhood obesity issue before becoming first lady; a doctor had warned her about her daughters’ weight.

At the White House, she decided to share her experience with the country and started by planting the first vegetable garden there in more than 60 years. That led the following year, in 2010, to the launch of her anti-childhood-obesity initiative, “Let’s Move.”

The first lady appealed to elected officials, food makers, sellers, restaurant chains and others to try to make healthy food more accessible. She lobbied lawmakers to add more fruit, vegetables and whole grains, and limit fat, sugar and sodium in the federal school lunch program.

That led to the first update to the program in decades, and for Mrs. Obama the process was akin to a crash course in Washington sausage-making. Mrs. Obama’s effort was not universally welcomed. Republicans in Congress wanted to reverse the rules. Others said Mrs. Obama was acting like the “food police.” Even the kids she wanted to help added to the backlash. Some students posted photos of lunches they found unappealing on Twitter with the hashtag #ThanksMichelleObama, or simply tossed the food into the trash.

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Mrs. Obama had won. But she would never again try to work closely with Congress on an issue. She chose instead to use her platform to press industry to change its ways.

It’s too early to know how Mrs. Obama’s efforts may affect childhood obesity rates long term, but advocates believe she helped change the national dialogue around healthy eating. And although incoming Republican President Donald Trump, a proud patron of McDonald’s and Kentucky Fried Chicken, has yet to comment on school meal regulations, advocates worry about the fate of Mrs. Obama’s effort under a White House and Congress that will be controlled by the GOP.

Reflecting on her childhood obesity work, Mrs. Obama said some people initially wondered why she would bother with such a “softball issue” but “now, all those challenges and criticisms are off the table.” She told talk-show host Rachael Ray that “at least we’ve become very aware as a society that this is one of our most important health issues.”

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Mrs. Obama’s push to put the country on a health kick extended to exercise — and she made herself exhibit A.

To promote “Let’s Move,” the first lady often donned athletic wear and ran around with kids at sports clinics, some on the South Lawn. She twirled a hula hoop around her waist 142 times and kick-boxed in a video of the gym workout that helped tone the upper arms she showed off regularly, as in her official White House photo.

She did pushups with Ellen DeGeneres, raced in a potato sack against late-night TV’s Jimmy Fallon in the East Room and shimmied with a turnip in a brief video popular on social media — all to show that exercise can be fun.

“I’m pretty much willing to make a complete fool of myself to get our kids moving,” she once said.

Instead of going the fool’s route, Mrs. Obama turned herself into a fitness guru and a figure significantly more popular than her husband.

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First lady was never a position Mrs. Obama imagined for herself, given her modest upbringing, her distaste for politics and having never seen her skin color on a U.S. president and first lady.

Her early aversion to politics developed while watching her father navigate Chicago politics for his job with the city water department, and was reinforced by her husband’s pursuit of a political career. Both Obamas have said his political ambition had strained their marriage and family.

Once in the White House, Mrs. Obama vowed to protect her then 10- and 7-year-old daughters’ right to a normal childhood. She declared being “mom in chief” to Malia and Sasha as her priority, irking women who hoped the first lady might be less constrained by stereotypes.

She showed few signs of trying to push those boundaries.

Mrs. Obama was an enthusiastic White House hostess. She rarely spoke about issues that were outside of her portfolio. She crafted her public schedule around her daughters’ activities and limited her travel so she could spend time with them.

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The Obamas’ parenting style — often described by both Obamas as warm, but strict — made them role models on that front, a point of pride, particularly in the African-American community.

“We have heard no Obama children drama,” said Ingrid Saunders Jones, national chairwoman of the National Council of Negro Women.

Mrs. Obama didn’t really begin to open up about the historic nature of her service as the first black woman to become first lady until the end of the presidency was in sight. She mostly addressed the subject in interviews when she was asked to reflect about it, and discussed how important it was for children to see a black president and first lady.

Longtime friend and White House adviser Valerie Jarrett said Mrs. Obama was often reluctant to talk about such matters earlier because she wanted her legacy to be more than just her place in history.

“Her goal is not what she is, but what she does,” Jarrett said.

Michelle Obama: Watching the 2016 Presidential Election Was ‘Painful’

In the final weeks of the presidential race, Mrs. Obama set aside her distaste for politics to wage one last campaign, an ultimately futile attempt to help elect Democrat Hillary Clinton. She quickly became one of most passionate Democratic voices opposing Trump and calling him out for “bragging about sexually assaulting women” in comments caught on a 2005 video.

“I know it’s a campaign, but this isn’t about politics,” she said at a Clinton rally shown live on cable TV news, rare exposure for a first lady in a campaign. If Trump’s past words are “painful to us as grown women,” she asked, “what do you think this is doing to our children?”

It was yet another moment when Mrs. Obama again seemed to be following her path rather than precedent.