Wearing a sexy black and magenta mini-dress, Underwood kicked off the night with "Good Girl," mixing country with a blast of rock.
Pictures: ACM Awards 2012 red carpet
Pictures: ACM Awards 2012 show highlights
Pictures: ACM Awards 2012 press room
ACM Awards 2012 winners
Eli Young Band took home the first award of the evening scoring song of the year for "Crazy Girl." The quartet beat out country heavyweights, including Lady Antebellum and Kenny Chesney. "I have dreamed this moment my whole life and I can speak for the rest of these guys up here that they have, too," lead singer Mike Eli told the crowd.
The album of the year award went to Lambert for her 2011 release, "Four the Record." Lambert looked surprised upon hearing her name. "Seriously?" Lambert said when she hit the stage. She added, "Every record is amazing, and I'm so thrilled. I will never take this trophy for granted. My albums are my babies. It's what makes me wake up in the mornings."
Lambert also won her third consecutive female vocalist of the year award. Her husband, ACM host Blake Shelton, scored male vocalist of the year.
Taylor Swift won her second straight entertainer of the year trophy at the ACMs. She bested her all-male competitors, including four-time winner Kenny Chesney, along with Brad Paisley, Shelton and Aldean.
While accepting her award, Swift took a moment to say hello to Kevin McGuire and his family. McGuire, a high school football player and Swift fan who has cancer, was to have been Swift's date to the awards, but he was hospitalized and couldn't make it.
"He's not here but I promised him I would give him a shoutout," Swift said of the Somerdale, N.J., resident.
The members of KISS, sporting full makeup, almost looked out of place as the rockers announced Lady Antebellum as winners of vocal group of the year. The country trio thanked fans, family and country radio for the win.
The night's single record of the year went to Aldean and Clarkson for their hit duet, "Don't You Wanna Stay." The pair's collaboration also won vocal event of the year.
"I kind of thought it needed a female vocal on it and Kelly was the first name I threw out," Aldean told the audience when he hit the stage. "And I'm really glad I did because she came in and made a great song, and just took it to a great place."
Added Clarkson: "Thank y'all so much for welcoming me from the pop world!"
"American Idol" alum Scotty McCreery won the fan-voted new artist of the year honor.
"Two and a Half Men" star Ashton Kutcher took the stage to announce female vocalist of the year. Wearing a cowboy hat, the newly single actor showed off his singing skills by belting out part of George Strait's "I Cross My Heart" before announcing Lambert as the winner.
Some awards were handed out before the televised show on Sunday night, including Toby Keith's "Red Solo Cup," which scored video of the year. His rowdy live version of the song got the ACM audience members out of their seats Sunday night at the MGM Grand Garden Arena.
Husband and wife duo, Thompson Square, meanwhile, was given the vocal duo or the year award during a concert Saturday night, topping Sugarland.
Chesney went into the night with nine nominations, while Aldean had six.
The evening's co-hosts Shelton and Reba McEntire didn't waste any time before they started cracking jokes. In their opening, the pair called out everyone from Newt Gingrich and Mitt Romney to Taylor Swift and Tim Tebow. Shelton questioned "rumors" about Swift going on a date with the NFL star. The humor continued through the night.
The awards ceremony was chock-full of performances by everyone from Lionel Richie and The Band Perry to Keith Urban and Sara Evans. Viewers didn't see an award announcement until more than a half-hour into the show.
Actor-comedian-banjo player Steve Martin joined Rascal Flatts on "Banjo" after a brief tribute to country pioneer Earl Scruggs, who died last week.
Martina McBride and Train's Pat Monahan teamed up to help New Jersey couple Christina Davidson and Frank Tucci get married on stage while they sang "Marry Me."
Among the stars who strutted on the red carpet included The Band Perry, Eli Young Band and Underwood, who arrived in a long strapless nude-colored dress with silver details. She said she kept stepping on the hem of her own dress. Jennifer Nettles of Sugarland described her custom Max Azria dress color as "watermelon Jolly Rancher."
