A fine second half display sees Gillingham leap out of the drop zone after just 48 hours as Southend take a step closer to League Two.
After the weekend's defeat at Oldham, Mark Stimson welcomed his captain Barry Fuller back to the starting line-up, together with Dennis Oli who partnered Rene Howe in attack.
In a nervy first half of few clear cut chances, the home side set out with the bit between their teeth and were unlucky not to take an early lead when Jackman's third minute free-kick fizzed wide of Mildenhall's left-hand upright.
The Shrimpers responded well and gave their hosts an almighty scare on 13 minutes as Simon Francis hooked Herd's cross goalwards, only to see Alan Julian reach back and palm it to safety at full stretch.
A mix-up in the visitors defence allowed Rene Howe in behind the Shrimpers back four ten minutes later but the powerful forward rolled his finish wide of Mildenhall's back post.
Good hold-up play by Oli in midfield allowed John Nutter to ghost in behind the Shrimpers defence ten minutes before the interval but his low shot was comfortably blocked.
In the 41st minute Gillingham missed a golden opportunity as Danny Jackman's blocked shot fell at the feet of Howe. The forward contrived to hit his shot straight at the keeper and watched on in horror as the
Shrimpers scrambled the deflection to safety.
Attacking the Rainham End in the second half, the Gills looked a livelier side after the restart and took the lead seconds after half time. A long ball found Curtis Weston on the right of goal, the midfielder's effort deflected off Mildenhall and was poked over the line by the lurking Dennis Oli.
Soon after Andy Barcham opened up the Southend defence with a surging run, he cut inside to shoot but was unlucky to see his drive deflected behind for a corner.
Oli came close to adding a second in the 53rd minute, good work by Howe allowed the flying forward to run into the box but his shot came off the leg of Francis and hit the crossbar. Seconds later Mildenhall saved superbly to deny Howe from close range again.
Adam Miller was equally profligate on the hour after a sensational run and cross from Barcham, the midfielder arrived unmarked at the back post and somehow sliced wide and into the side netting.
Twenty minutes from time Stimson sent on Simeon Jackson and the leading striker repaid his manager two minutes later. Jackson raced on to John Nutter's deep ball out of defence and played in Rene Howe who rifled his first goal for the club across goal and into the bottom left corner.
In the 79th minute Alan Julian dived to his left to make an acrobatic save from Southend sub Matt Paterson's curling effort.
Five minutes from full-time, Andy Barcham secured the points with a beautifully crafted goal that saw Barcham play a neat one-two with Weston before planting a rising drive into the roof of the net.
Attendance: 7657 (927 visitors)
Gillingham: Alan Julian, Barry Fuller, Garry Richards, Darren Dennehy, John Nutter, Adam Miller, Curtis Weston (Kevin Maher 89), Danny Jackman, Andy Barcham, Rene Howe (Stuart Lewis 89), Dennis Oli (Simeon Jackson 70).
Subs not used: Simon Royce, Mark Bentley, Chris Dickson, Josh Gowling.
Southend United: Steve Mildenhall, Simon Francis, Jean-Francois Christophe (Stuart O'Keefe 74), Pat Baldwin, Adam Barrett
, Anthony Grant, Osei Sankofa, Scott Vernon, Johnny Herd, Francis Laurent (Harry Crawford 77), Scott Spencer (Matt Paterson 65).
Subs not used: Scott Malone, Jean-Yves M'Voto, Daniel Bentley.