The Oklahoma native received more viewer votes than runner-up Bice, of Alabama, after Tuesday's final round, snaring the title and a record contract.
It was the first time a country singer has won the talent competition.
"This season, I knew somebody different was going to win," Underwood told CBS News Early Show National Correspondent Hattie Kauffman.
Ratings for last night's crowning appeared to be slightly higher than they were last year. The early numbers indicate that nearly 29.5 million people watched Underwood take the top prize — compared to just under 29 million who tuned in a year ago.
A tearful Underwood choked out a brief "thank you," then spoke with a song. She reprised "Inside Your Heaven," which both she and Bice performed in Tuesday's final round. The judges thought Bice out-sang her; the voting audience obviously disagreed.
Bice might have lost out to Underwood, but the one who's really upset is Corey Clark.
Clark, the former contestant who says he had an affair with judge Paula Abdul during the 2003 competition, didn't like the skit making light of the situation. On Wednesday night's show, TV newsman Steve Edwards presented a satirical report entitled "Bad Judgment" that purported to reveal judge Simon Cowell's "secret relationship."
The twist, though, was that the self-centered Cowell was all along in love with himself.
"We're reminded once again last night in front of millions of viewers that not only are they not taking it seriously, but they don't feel as though we should take it seriously," Clark's spokesman, Jed Wallace, told AP Radio on Thursday.
"It's simply baffling to me that Fox would actually resurrect this controversy by doing something like that," said Wallace, calling it a "slap in the face."
Backstage, Underwood savored her victory and what could follow.
"This is the best night of my life. And it's going to get better," she said.
The singer added that she's ready for the pressures of a music career and won't disappoint her fans: Country music is where she intends to make her name.
"I love country music. That's where my heart is," she told Kauffman. "That's the kind of music I want to be in, those are the kind of people I want to be around."
Bice said he was satisfied with the outcome and the experience.
"I never thought I was going to make it this far. I'm grateful that America kept me here," he said. "Man, I was just trying to get some better gigs around town!"
He had entered the "Idol" contest on a bet with his mom, expecting only that the exposure might get him better gigs back home. Turns out the payoff will be bigger.