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.
Leanne Mitchell has won the first series of BBC singing competition The Voice.
Mitchell, mentored by Sir Tom Jones, looked stunned after the result of the public vote was announced in tonight's live show and said: "I am starting to believe in myself."
She beat Tyler James, mentored by Will.i.am, and Bo Bruce, mentored by Danny O'Donoghue.Vince Kidd, coached by Jessie J, was eliminated earlier in the show.