The judges said Alaina, who producers said earlier Tuesday lost her voice during rehearsals, had won the night after performing the song "Like My Mother Does," which would be her single if she wins.
If Alaina had been unable to sing, producers said Haley Reinhart, who was eliminated last week, would have taken her place.
McCreery, the 17-year-old baseball-playing high school student of Garner, N.C., had a good night as well, but didn't get as much praise from the judge's during his potential single "I Love You This Much."
McCreery wowed earlier in the night with performances of Montgomery Gentry's "Gone" and George Straight's "Check Yes or No."
Alaina, the 16-year-old cheerleading high school student of Rossville, Ga., took on Carrie Underwood's "Flat on the Floor" and "Maybe It Was Memphis" by Pam Tillis.
Judges called the first round for McCreery and the second and third rounds for Alaina.
After the performances, McCreery told WRAL News that he was happy it was over.
"I'm so glad it's over. It's been such a stressful week for us both. We went out there and gave it our best. It's in America's hands now," he said.
The pair have both delivered consistently smooth performances since their attention-grabbing auditions when McCreery wowed with his deep register on Josh Turner's "Your Man" and Alaina impressed with her soaring range on Faith Hill's "Like We Never Loved At All." McCreery never appeared as one of the low vote-getters, and Alaina did only once three weeks ago.
The duo have managed to outlast more seasoned and experimental finalists, such as 22-year-old songstress Pia Toscano of Howard Beach, N.Y., 22-year-old rocker James Durbin of Santa Cruz, Calif., and 20-year-old Haley Reinhart of Wheeling, Ill.
Throughout the competition, McCreery has maintained his old-school country style with renditions of such songs as Travis Tritt's "Can I Trust You with My Heart" and Garth Brooks' "The River," while Alaina has presented herself as a potential pop-country princess with performances of Shania Twain's "Any Man of Mine" and Miley Cyrus' "The Climb."
The show's judges — Randy Jackson, Jennifer Lopez and Steven Tyler — have equally heaped praise on the upstarts. However, without ever facing elimination this season, McCreery is expected take the title over Alaina after Tuesday's installment with a bigger base of voting fans, joining the likes of past "Idol" winners like David Cook, Kris Allen and Lee DeWyze.
McCreery said Monday that he has tried to remain humble despite the praise he has received. He said he hoped to win the title for the people of his hometown.
"To all the people back home, I'm going to give it my best. I'm going to try to bring it back home to Garner. I'm doing all this for them. They are the ones (who) got me here," McCreery said Monday alongside fellow finalist Lauren Alaina.
The Town of Garner held a viewing party Tuesday night at McCreery's school, Garner Magnet High School.
"I've known him since eighth grade. He's a very good singer and I'm proud of him," fan Briana Wells said.
Shortly before the party started, Garner Magnet received a check for $10,000 from Ford – a major "American Idol" sponsor.
A viewing party for the "American Idol" finale will be held Wednesday at the RBC Center. Admission is free. Parking will be $10. Doors open at 6:30 p.m. If you can't make it to Raleigh, you can watch the party LIVE on WRAL.com.