Centre Adam Powell yesterday put pen to paper on a new deal and England Under-20 international Mercey, 19, has quickly followed suit.
Director of rugby Alan Gaffney said: "Tom is one of the most outstanding front-row talents I've seen in my career.
"He has been a vital member of the first-team squad this season and has turned in some excellent performances.
"Looking back at our EDF Energy Cup match against Wasps in December, Tom came up against Tim Payne of Wasps - a player who this week has been selected for the England side.
I would have to say that on that day, Tom got the better of him.
"Looking at the fact that he's still only 19 years old and already capable of handling himself against some of the best props in the competition, he's got the potential to be a fantastic player for Saracens."