The 17-year-old gymnast was named "Dancing With the Stars" champion Tuesday during the season finale of the hit ABC show.
The winning couple held the mirrorball trophy aloft as the rest of the contestants rushed out to greet them, hoisting them into the air as sparkling confetti flew.
"This has been the best experience ever," Johnson said after her victory. "I've grown so much."
Johnson was tied with fellow finalist Gilles Marini after Melissa Rycroft was dismissed earlier in Tuesday's show.
All three finalists performed one last dance, each earning a perfect score of 30 from the judges. Viewer votes are combined with judges' scores to determine the "Dancing" champ.
Show host Tom Bergeron said "less than 1 percent" separated the two finalists.
"This has been, without a doubt the best season ever," said head judge Len Goodman. "We've got the three best dancers here in the final."
"I'm so happy for Shawn and Mark. They worked so hard," Marini said, referring also to Mark Ballas, Johnson's professional dance partner.
Johnson had a scare in March when a 34-year-old man allegedly tried to break onto the "Dancing With the Stars" set, apparently to meet her. Police say Robert O'Ryan, who remains in custody on a stalking charge, jumped a fence at the studio lot after driving from Florida in a car loaded with guns, duct tape and love letters to Johnson. Filings by her parents said the incident caused her to fear for her safety.
But Johnson shook off the incident and used her athleticism to better effect with each passing week, looking less like a rigid, underaged gymnast and more like a powerful yet graceful dancer as she gained confidence.
On Tuesday, former contestant Jeffrey Ross came back for a little comic relief, roasting the three finalists, calling them, "the underage, the unwed and the unpronounceable."
Tuesday's season finale also featured a performance by Lady Gaga and the finale of the new pro-dancer competition. Anna Damidova competed with three other professional dancers for a spot on next season's "Dancing With the Stars."
Previously dismissed celebrities - including Apple co-founder Steve Wozniak, rodeo champ Ty Murray, funnyman Steve-O, singer Belinda Carlisle, rapper Lil Kim, former football star Lawrence Taylor, country singer Chuck Wicks, reality star Holly Madison, and actors David Alan Grier and Denise Richards - returned for a last dance.
While Johnson was paid increasingly by the episode, she got nothing extra for taking the mirroball trophy, according to court documents filed in Los Angeles.
Because she's under 18, Johnson had to have a judge approve a minor's contract to participate in "Dancing." That agreement spelled out that Johnson receives $125,000 for being a contestant, and made significantly more per week the longer she lasted: For weeks three and four, Johnson received $10,000 per week; $20,000 per week for weeks five, six and seven; $30,000 for weeks eight and nine; and for the final two weeks, she received $50,000 a week - but no winning bonus.