The singer's winning track 'Run To You' - which she performed during Saturday's (June 3) live final - has landed at number 27 in the UK midweek chart.
Although the song hasn't been officially branded a winner's single, Mitchell was the only contestant from the final to receive a formal release via digital outlets.
The disappointing chart position comes despite The Voice UK live final being seen by a peak audience of 8.7 million on BBC One.
However, the track's release was not highly publicised during The Voice UK final and hasn't been released physically.
Meanwhile, runner-up Bo Bruce enters the UK midweek album chart at number 19 with her EP Search The Night.
Bruce - who was hotly tipped to win the series - recorded the collection of music before entering the show in 2010 and saw the EP vault up the iTunes chart following her defeat on Saturday.
As part of her Voice UK prize, Mitchell has been handed a £100,000 recording contract with Universal Music and is expected to release her debut album later this year.
Watch Leanne Mitchell perform 'Run To You' on The Voice UK live final below: