The couple, who previously competed in different seasons of the Fox competition, were invited up on stage by host Ryan Seacrest during last night's episode.
"You guys are too cute," he said. "Come on up here on the stage... It's amazing what can happen out of this show."
Seacrest asked the couple about moving in together, before Young said: "This has always been home for us and I felt that this would be the perfect place to ask a simple question."
As the audience cheered and the lights were dimmed, Young said: "Diana, we have conquered Broadway together. We have created new music together. We have an amazing group of people around us and with the help of David Webb jewelery, I have a way to make this fun last forever."
Asking: "You ready?" Young then got down on one knee as the audience cheered and DeGarmo laughed tearfully.
"I love you," Young said. "You're my best friend. I want to make this last forever and I will do anything in my power to have the most unimaginable and amazing life together if you will have me. Diana Nicole DeGarmo, will you marry me?"
DeGarmo nodded and the pair then kissed as Seacrest and the crowd applauded, before Young put the engagement ring on DeGarmo's finger.
Phillip Phillips and Jessica Sanchez battled it out in the American Idol finale, which crowned one of them the winner.
Photo gallery - American Idol grand finale (contains spoilers):