The 20-year-old, who has come through the ranks at the Hawthorns, has featured just once for the Baggies this season - in the 4-1 Carling Cup victory at Bournemouth in August.
Saddlers manager Dean Smith was delighted to have secured Mantom's services.
He told Walsall's official website: "He's a young lad that I have kept an eye on over the past couple of years. He has matured into a very good player and is training regularly with the first team at Albion.
"Having spoken to the people there and having watched him in action myself, I know that he is ready for the challenge and I am looking forward to working with him here.
"He will be a useful acquisition for us. He's a ball player who is comfortable on the ball and plays with a maturity beyond his years."
Mantom could make his debut in the Saddlers' npower League Two clash with Sheffield United on Tuesday night.