The England Under-19 winger picked his spot in the 61st minute after a low shot by Mark Wright had been well saved by unlucky Wrexham keeper Anthony Williams.
It was fitting that Demontagnac should score the winner, because he had been the main threat to the Wrexham defence in the first half in which the visitors were the better side for long periods.
Neil Roberts had a 16th minute shot deflected over when Danny Williams headed down a throw in from Simon Spender, but at the other end Walsall's Anthony Gerrard had a good chance in the 36th minute only to head wide at the far post from Wright's in-swinging corner.
The best chance of the opening period fell to Wrexham's Lee McEvilly in the 44th minute. An error by Walsall's makeshift fullback Tony Bedeau let in Chris Llewellyn whose cross to the far post was headed wide by the Wrexham striker.
Walsall were much improved after the interval with Wrexham posing little threat to home keeper Clayton Ince.
Hector Sam had two chances for Walsall, but the substitute striker headed just over from a Dean Keates' corner in the 78th minute and then fired wide from an even better opportunity laid on by Martin Butler seven minutes later.
Nevertheless, Walsall moved to the top of the League Two table thanks to having a better goal difference than Hartlepool, and duly deserved to celebrate a fine season with a well-received lap of honour.