The French striker joins Sean Morrison and Brett Williams at the Madjeski Stadium, initially on loan, so he can play in the Royals clash with Hull although the formalities of a permanent transfer will be ironed out on Monday.
Manset has been in blistering form for Hereford of late, with 13 goals in 16 games and his presence is sure to help Reading ignite their npower Championship promotion push.
Reading boss Brian McDermott told the club's official website: "Mathieu first came to our attention in May 2008 when we saw him playing for Havre against Lyon and we've tracked his development ever since.
"In the last few months his goalscoring record speaks for itself, there was lots of interest from Championship clubs and once again we're very lucky, pleased and excited he has joined us."
Manset added: "I feel good. I'm so glad to be here. This is a big opportunity for me. I am young, so I need to take my chance. This move could be a big thing."