Demontagnac, starting his first game since early last season, demonstrated skill and goalscoring flair which made it difficult to understand why he has been continually over-looked by the Walsall management.
The turning point of the high scoring game occurred when Walsall capitalised upon two bad defensive mistakes to get back on track after being derailed at Crewe last week.
The Saddlers sealed the game in the space of two minutes at the start of the second period after the first half had been an evenly-contested affair.
Southend had previously displayed plenty of promising play but were shocked when Anthony Grant turned in a left-wing cross from Marco Reich to enable Walsall to regain the lead.
Before they had time to recover Southend found themselves in further arrears centre half Peter Clarke left a defensive header back to Blues keeper Steve Mildenhall well short and Demontagnac nipped in to lash his shot past him.
The new-look Walsall side took a long time to settle and this allowed Southend several opportunities to pose problems to the home side.
Fortunately Anthony Gerrard, linked with a possible move to Reading before the transfer deadline, was superb in defence and kept the Saddlers in the match.
Demontagnac highlighted his possible threat when he hit the base of the post with a speculative shot which encouraged Walsall to mount another attack which resulted in Reich chesting down a Chris Palmer cross before hammering home a powerful drive.
Southend, who suffered their second successive defeat, hit back to equalise in the 33rd minute. A long cross from the right was headed back by Alex Revell into the path of Hal Robson-Kanu to score with a low shot.
But Southend were caught napping at the start of the second period when they were hit by Walsall's two-goal blast. Afterwards it was always an uphill struggle despite some encouraging work from Lee Sawyer.
Demontagnac hurled himself forward to head home a Palmer cross in the 69th minute for his second goal but his most spectacular effort came in the 84th minute when he cheekily scored with a cross from the left. There were not too many kind words for Mildenhall from the Southend support for that one.
Barnard pulled back an 87th-minute goal for Southend but all the honours belonged to Demontagnac.