He said: “The Under 19s have a lot of players that need game time and Tony Rea suggested it might be a good idea to come back and play for the club I started at. I’m really excited to player with some of the players that I used to play with when I came through the junior ranks at the Stags.
Sarginson, 19, was born in Australia but raised in Hemel Hempstead, starting with the Stags before moving on to Harlequins, for whom he made his debut against Gateshead in May 2011.
He has since gone on to score 11 tries in 28 appearances for the London club.
Hemel coach Troy Perkins said: “It is great to have Dan dual registered with us. I am expecting him to be playing Super League for the majority of this season but if he is coming back from injury like he is at the moment he will be a great addition to our squad.
“He is a very good player and, whenever he trains for us, he brings a professional attitude that all the other players should be aspiring to have.”