James Brown has signed a new contract which will keep him at The Den until at least the summer of 2020.
The 19-year-old has made one appearance for The Lions since signing a professional contract in April last year.
The right-back has since established himself as a key member of Carlisle United's defence during a loan spell this season, making 14 appearances including in their 4-0 win over Yeovil last week.
Brown is delighted to have extended his contract, telling millwallfc.co.uk: "I knew my contract was up at the end of the season and it's really nice to get it all sorted out."
"It's great to have that security and now it's just about focusing on my football.
"I have been able to learn a lot during my loan spell at Carlisle. Playing regular first-team football has been the main thing for me and playing in front of fans. It's helping my development as a player."