"I want to talk to people about how to set a goal and achieve it," she told The Associated Press after the show. "Because I just achieved my goal."
Stewart edged out first runner-up Leah Laviano of Mississippi and Tiffany Andrade of New Jersey.
Miss USA 2007 Rachel Smith relinquished the crown - and the posh New York apartment that comes with it - in a show aired live by NBC with hosts Donny and Marie Osmond from the Planet Hollywood Resort & Casino on the Las Vegas Strip. The network co-owns the parent Miss Universe Organization with Donald Trump.
Smith, a former Miss Tennessee USA, said she was headed to Hollywood.
Stewart was headed for the publicity circuit. She said she was eager to travel and spread her message of self-improvement to young women. She noted she was one of only a handful of black woman crowned Miss USA in the pageant's 57-year history.
"I think the United States is coming together," she said, citing the historic presidential candidacies of Barack Obama and Hillary Clinton. She declined to name her preference.
"I don't know, we'll see. Fundamentally, I'm a Democrat," she said.
Contestants from all 50 states and the District of Columbia have been in Las Vegas for nearly three weeks, rehearsing and hyping the 57th annual pageant. After stints in Baltimore and Los Angeles, organizers billed the new venue as a city beloved by the event's international audience.
The pageant tried to showed off its edge, featuring a grinding live rock performance by the band Finger Eleven, music from Rihanna, and contestants in barely there black bikinis and faux-fur coats.
Donny and Marie kept up a steady stream of sibling banter even while swiftly whittling the field.
Donny Osmond told the losers to "put on a poker face" as he sent them home.
"Or use Botox; then it won't move," Marie quipped.
Stewart will compete in the Miss Universe pageant in Vietnam in July. She also becomes a spokeswoman for breast and ovarian cancer awareness and other causes, while traveling to promote the organization.
Miss USA contestants are scored in three categories: swimsuit, evening gown and interview. Miss Alaska USA, Courtney Erin Carroll, was chosen "Miss Photogenic USA" based on voting at the organization's Web site. The other contestants named Miss Ohio USA, Monica Day, "Miss Congeniality." Unlike the rival Miss America, Miss USA contestants are not asked to perform a talent.
Smith's year on the throne has been marked by fewer racy headlines than her 2006 predecessor, Tara Conner. Conner's underage drinking landed her in rehab and sparked a media circus documenting her fall from grace.
Smith's low point was a fall onstage during the evening gown competition of the Miss Universe pageant in Mexico City. She was booed by the crowd and Miss Japan won the title.
A bigger blunder this year belonged to Miss California USA organizers. The judges crowned the wrong queen in their November contest and reversed it days later, saying Raquel Beezley, of Barstow, was the victim of a vote tabulation error. Dethroned Miss Los Angeles, Christina Silva, a Hispanic woman, has filed a lawsuit alleging racial bias.
Asked to comment on the lawsuit, Stewart was whisked away by organizers. The Miss Universe Organization would not comment on the matter.
The panel of judges for Friday's pageant included Heather Mills, model and former wife of Paul McCartney; comedian Rob Schneider; Olympic gold-medal swimmer Amanda Beard; and Christian Siriano, winner of Bravo's fashion reality series, "Project Runway."