The Saddlers struggled during a disappointing first half, but demonstrated their promotion credentials with a Dean Keates double and goals from Mark Wright and Anthony Gerrard after the interval.
Mansfield boss Peter Shirtliff got his tactics right in the first half and his five-man midfield out-muscled the hosts to take control of the game.
The Stags also created the best chance of the opening 45 minutes, but Callum Lloyd drove over in the 18th minute from a cross by Matt Hamshaw.
At half-time Walsall's shrewd manager Richard Money made a change.
He brought off ineffective striker Hector Sam and brought on another midfielder in Craig Pead.
The match burst into life in the 50th minute when Lloyd forced a fine save from Walsall keeper Clayton Ince after good work by Alex Baptise and Richard Barker. Play quickly switched direction and the hosts' Daniel Fox brought a save from Jason White.
The opening goal followed moments later when winger Wright swept the ball into an empty net after a shot by Pead had been blocked.
Ince then made another impressive save from Alan Sheehan's free-kick to keep Walsall in front, and they increased that lead in the 62nd minute when White allowed a free-kick from Keates to wriggle under his body.
Walsall's man of the match Keates, one of the smallest men on the field, added another goal with a header from a cross by Wright in the 76th minute.
The rout was completed in added time with Keates in the action again, this time crossing the ball for Gerrard to score from close range.
Mansfield were largely ineffective against Walsall's second-half onslaught, but Ince had to use all his height to make a magnificent interception from a cross by Hamshaw in the 60th minute, and five minutes later substitute Danny Reet drove wide after Barker headed the ball down.
In the dying minutes, Mansfield had a chance to get a consolation goal but Ince made a low save from a shot by Hamshaw.