Each contestant had to sing three songs; their own favourite, a choice from Simon Fuller, and the winner's single 'No Boundaries'. The track was co-written by Idol judge Kara DioGuardi, along with Cathy Dennis and Mitch Allan.
After winning, Allen told host Ryan Seacrest: "It feels good, but Adam deserves this. I don't even know what to feel right now. This is crazy."
As well as his performance of the winner's song, Allen won the vote with his take on Bill Withers's 'Ain't No Sunshine' and Marvin Gaye's 'What's Going On'.
Favourite Lambert had to settle for second place after singing 'Mad World' by Tears For Fears and Sam Cooke's 'A Change Is Gonna Come'.