Sir Tom Jones has announced the postponement of his upcoming US tour due to "medical advice".
The pair will be joined on the judging panel by Oscar-winning actress and singer Jennifer Hudson and rock star Gavin Rossdale.
The singer said that dealing with the death of his wife Linda was "the hardest thing I've ever done in my life".
After the result was announced, Sir Tom said: "This confirms that there is justice in this world."
Mitchell, who has won a recording contract, first impressed the judges in the blind audition stage of the show when she sang Beyonce's If I Were A Boy.
Tonight she delivered strong performances of It's A Man's Man's Man's World by James Brown and Run To You by Whitney Houston.
She also sang Mama Told Me Not To Come by Eric Burdon and The Animals in a duet with Sir Tom, a song he had previously covered with the Stereophonics.
Sir Tom Jones has revealed he wants to play a "bad guy" after making his acting debut playing an ageing teddy boy.
The singer, who is currently appearing as a coach in BBC1 talent show The Voice, said his role in Sky Arts show King of the Teds was not "far removed" from how his life could have been.
He plays Ron, a redundant bottle factory worker who was once a top teddy boy in his youth, in the drama alongside Alison Steadman and Brenda Blethyn.
Sir Tom said: "I was a teddy boy and I was singing so if I hadn't had a hit record that could have been me."
The singer said acting was more difficult than his role on The Voice which was "still music" but that he would like to do more.
He said: "I wouldn't mind playing a bad guy, again it wouldn't be that far removed."
Sir Tom, who turns 72 in June, previously took a brief cameo role in Tim Burton's comedy Mars Attacks, playing himself.