The 17-year-old, who was never in the bottom two, beat fellow contestant Lauren Alaina, 16, in the show's first all-country finale.
The two finalists are the youngest duo to compete in the long-running show, with McCreery becoming the second country singer to win the competition after Carrie Underwood took the crown in 2005.
"Never in my wildest dreams," said McCreery in disbelief. "I got to thank the Lord first. He got me here."
The results were announced following a two-hour star-studded live broadcast that included performances from Lady Gaga, Beyonce, Bono and new judges Jennifer Lopez and Aerosmith frontman Steven Tyler.
American Idol continues its reign as the most-watched TV show with nearly three-quarters of a billion votes cast over the course of the tenth season, host Ryan Seacrest announced.
McCreery is the fourth male in a row to win the Fox TV contest, with 2007's Jordin Sparks, 17, the last female Idol.
Kelly Clarkson and Carrie Underwood, two of the four female winners, have proved the victors with the most successful recording careers, with Underwood recently becoming Idol's top earner by surpassing season one champion Clarkson.
