The 17-year-old, who hails from Wirral, has been with the Reds since the Under-7s level.
Pinnington played for the Under-18s for the first time last year as a defender.
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He has also been involved with Barry Lewtas' Under-21 side at Melwood this season.
Pinnington impressed in the Bristol Street Motors Trophy campaign this season.
The club have confirmed that he has put pen to paper on a professional deal for the first time.