Mount, 20, joined Chelsea in 2005, aged six, and was part of the side that won the FA Youth Cup in 2016 and 2017.
The England Under-21 international spent last season on loan at Derby, then managed by new Chelsea boss Frank Lampard, helping the club reach the Championship play-off final according to bbc.com.
“It’s a massively proud moment for myself and my family,” said Mount.
“It was always my goal to play for the first team one day. I’m really looking forward to the coming season now and working hard to have an impact here at Chelsea.
“I’ve been at the club for a long time already and hopefully I’ll stay for a long time to come.”
Mount was among the scorers as Chelsea beat Irish side St Patrick’s Athletic in their second pre-season friendly of the summer in Dublin on Saturday.