Despite receiving universal praise from the judges for her performance of 'Superstar' from the musical Jesus Christ Superstar, the Irish barmaid found herself in the bottom two with Syesha Mercado after a public vote.
Judge Simon Cowell admitted his praise for Smithson's performance may have hindered her chances of progressing.
"I apologise for giving you a compliment last night, the kiss of death," he said after Smithson's exit. "You can leave with your head held high."
Sir Andrew Lloyd Webber acted as guest mentor, with contestants performing songs from his stage musicals.
Brooke White forgot her lines while performing 'You Must Love Me' from Evita and Jason Castro's rendition of 'Memory' from Cats was described as a "train wreck" by Randy Jackson.
The remaining finalists, Mercado, White, Castro, David Cook and David Archuleta, will next week be mentored by Neil Diamond.