The 18-year-old attacking midfielder – who signed his first pro deal last year – is now tied to the Blues until 2021.
He successfully progressed through the club’s academy and is a regular at Premier League 2 level.
Mount wrote on Instagram: “Pleased and thankful to have signed a new 4 year contract with @chelseafc.
A sentence at the bottom of Mount’s profile on the club’s website read: “In April 2017, he signed a new contract that commits him to the club until the summer of 2021.”
Mount has also skippered Jody Morris’ Under-18s this term, leading them to another FA Youth Cup final.
He got his hands on silverware last term, helping the Blues to the FA Youth Cup and UEFA Youth League.