Mantom joined the Saddlers in January from West Brom and has been a key performer for Dean Smith's side.
"I am really pleased that Sam has committed his future to us," Smith told the club's website. "Since he first arrived here, he's been a permanent fixture in the team.
"He's got better and better and is a player with enormous potential. I am looking forward to continuing to work with him and develop him as we develop as a club."
The former England Under-17 international and his team-mates will host Peterborough in Sky Bet League One on Monday night.