Tom Knowles' corner was bundled in at the far post by Monthe in the 91st minute to leave the Robins fans despairing but the 465 Saddlers travellers delighted.
It was probably just about deserved on second half possession and chances, Walsall having had two good chances to win the game before this, although they probably wouldn't have complained at a point.
At times throughout the game Walsall had to defend desperately but they stood firm, which Michael Flynn may have been just as pleased with as the fact they snatched it at the end.
Walsall were returning to action after a stop start December in which they had only played one game up to this point.
That was a 2-0 win at Doncaster, a result which meant The Saddlers were six games unbeaten and they would go on to make it seven, with two wins ground out on the road keeping them more than firmly in play off contention
Swindon manager Scott Lindsey had come under fire from the home fans in the last home game, but a win and a draw on the road had lifted his team into the play off places where Walsall want to be and the reception the team received on a cold but sunny day was warm, until the end of the game when boos once again rang out around The County Ground.
Walsall looked rusty early on struggling to get anything going and having to defend from the start against a Swindon team who looked keen to build on their play off ambitions and take advantage of being at home.
But there weren't many chances for either side in the first 15 minutes and in fact it was Walsall who went close on 18 minutes when Danny Johnson under pressure from a Swindon defender put the ball over the bar.
Tom Knowles on the right for Walsall was trying to be creative but getting little support in his efforts and what attacks Walsall had were seemingly easily broken down, with Donervon Daniels in action more than once to mop up pressure from the home side.
It finally told on 31 minutes when Ellis Iandolo opened the scoring for Swindon at the second attempt. Walsall were caught in possession and Ronan Darcy took the ball down the right flank, whipping it in for Iandolo whose initial shot was saved by Owen Evans. The rebound eventually came back for Iandolo and the ball found its way into the net.
As the half wore on towards the break it looked more than likely Saddlers were going to go in one goal behind but when Liam Bennett's shot was deflected for a corner on 45 minutes, Knowles went into action, His initial corner hit a defender but he gathered it, tried to cut inside and shot, with the ball going in off Remeao Hutton who was credited with an own goal.
The second half saw a similar pattern at the start, Swindon's Marcel Lavinier seeing a shot blocked but it was a lively byopening by the Robins. For Walsall it was the same pattern, with players having to be creative on their own to fashion an opportunity. Danny Johnson did just that with a run and turn inside to get a shot off which was deflected wide for a corner but the scrappy nature of the game was continuing.
Johnson was full of running and on 64 chased down a poor back pass which put Swindon goalkeeper Solomon Brynn under pressure the Middlesbrough loanee just managing to clear.
Michael Flynn introduced Connor Wilkinson and Jacob Maddox with 20 minutes left to try and give Walsall more zip and it almost paid dividends a few minutes later,. Wilkinson initiated an exchange of passes that saw Johnson bear down on goal in the area but he could only see his shot blocked by Brynn.
Wilkinson almost snatched it himself with a looping shot which just went wide of Brynn's post and Walsall saw the chance to win the game.
They eventually took it with the scrappiest of goals, once again as in the Carlisle cup game right at the end which will also be pleasing for Flynn as he has a side here that never knows when to say die and won't settle for second best. The fact they are making a habbit of coming back from a goal down to win the games will have also made his Christmas as they go into a busy programme which continues on Thursday at home to Stockport.
Walsall: (3-4-1-2) Evans; White; Gordon (Wilkinson 70); Daniels; Monthe; Kinsella; Knowles; Earing; Hutchinson (Maddox 70); Bennett; Johnson (for James-Taylor 85) Subs not used: Williams; Comley; James-Taylor; McEntee; Allen
Swindon (4-3-3): Brynn; Hutton; Iandolo (for Gladwin 55); Reed. Williams; Darcy; Blake-Tracy; Macdonald; Lavinier; Wakeling (for Hepburn-Murphy 55); Shade (for Roberts 78)
Subs not used: Copland; Gladwin; Adeloye; Khan; Minturn;