Mount inked a four-year deal at Stamford Bridge, keeping him at the club until 2021.
The former Purbrook Park schoolboy signed his first professional contract with the Blues on his 17th birthday, having been at the club since he was six.
He also trained with Pompey as a child.
Mount is Chelsea’s current under-18s captain and has led his side into the FA Youth Cup final.
The west London outfit will meet Manchester City in the final for a third straight year.
Chelsea won the FA Youth Cup last year, with Mount scoring in the 1-1 draw in the first leg away at City.
The Blues won 3-1 at home to bag the silverware.
The midfielder also played a pivotal role in Chelsea’s UEFA Youth League triumph last year.
As reported by The News, Mount has a dream to play at Fratton Park during his career.