The 23-year-old spent his formative youth years with Tottenham Hotspur before joining the Lions in 2015 in pursuit of regular football.
Archer has been a crucial part of Harris' team, which recently defeated Leicester City in the FA Cup to book a last-eight tie against Spurs.
Harris said: "Jordan came to Millwall because he was desperate to become a No1 at a young age.
"He has a hunger, an appetite to be better. When I left him out last season he was banging my door down - he doesn't like being dropped.
"He has now shown his quality. Like most players he had a little dip in form but when we have needed him to come up with big moments he has. He just wants to win, wants to play and he wanted to learn.
"He has been exceptional and is still a young man, but is very driven. Jordan is getting better but he would be the first to admit he still has a lot to learn. He has a big future at this club.
"We do our homework on signings and we wouldn't bring in someone who was larging it because he had been at a big club.
"The days of us taking a player because of his name or the size of the club he played for have gone. We only take those who we believe can fit into the mentality of our club.
"We spend a lot of man hours doing this and we miss out on good players at times because we feel they would not fit in our dressing room."