The 17-year-old was chosen to return over James Michael, Jonjo Kerr and 2 Shoes in tonight's show. Host Dermot O'Leary revealed that she received 48% of the vote.
All four were axed by their respective mentors Kelly Rowland, Gary Barlow, Louis Walsh and Tulisa Contostavlos in the first week of the live stages, but were given a reprieve following Frankie Cocozza's ejection on Tuesday (November 8).
When asked by O'Leary how it felt to be back in The X Factor, Lily replied: "Oh God, I've been so emotional backstage, I cannot believe I'm back in. Thank you everybody for believing in me."
She went on to describe the 48 hours since the announcement of the return twist as "the most stressful two days of my life, but so worth it".
Amelia sang 'The Show Must Go On' by Queen as her comeback performance, which received a hugely positive reaction from the audience and panellists.
"I don't know what to say, that was an amazing powerful vocal," Walsh said to her. "Welcome back, I think now you're going to get to show your proper talent. I want you to go far in the competition."
Contostavlos stated that Lily "deserve[d]" to return, having managed to hold her own against acts who "had weeks to progress and and perfect" themselves.
Barlow, meanwhile, described her performance as "absolutely sensational" and alluded to his own career with Take That by saying: "Let me tell you something from somebody who knows, the great British public love a comeback. Get ready."
Lily's mentor Rowland concluded the judging by excitedly announcing: "You mean some serious business and I love it! Welcome freakin' back!"
Lily then confessed to O'Leary that she had been "shaking" throughout her comeback performance.
"I'm so happy, I'm so grateful," she added. "I'm so happy to be back. It's just amazing."
The X Factor continues tomorrow (November 13) at 8pm on ITV1.
> X Factor Lady GaGa vs Queen week - Live Blog