The 19-year-old has put pen to paper on a three-year deal at The Hive London.
Mason-Clark, who has made 13 appearances so far this season, is a product of the Barnet FC Academy and made his first-team debut in October 2016.
Speaking on signing a new deal, Mason-Clark said:
“I’m happy to get this done – I need to keep improving, keep giving 100%.
“Coming up through the academy, it just shows that anyone can do it. I just need to stay positive and keep working as hard as I can.”
Manager John Still added:
“Since I’ve been here, I have been ever so impressed with Ephron. He’s a great trainer, he’s got a great attitude, he has always got a smile on his face and on top of that, he’s a very very talented young man.
“He was one of the players that, in the early stages, really bought into how we wanted to do things and he has just gone on from strength to strength.
“We’ve pushed a lot of the young players like Jack Taylor and Wes Fonguck, but Ephron is a little younger and we’re trying to take our time with him but it’s a great signing for the club and we are going to see a player who will go on and have a terrific career.”