Millwall booked a JPT Southern Area Final date with Oxford United as goals in either half from Steve Morison and Shaun Williams secured an excellent victory at Roots Hall.
Skipper Morison set the victory wheels in motion with a sweetly struck 34th minute goal and when midfielder Williams hit a second from distance on 65 minutes, The Lions were never in any real danger of relinquishing their lead.
Tuesday night's win, a result that sets up a two-leg Final with Oxford, was just the response manager Neil Harris had demanded from his players after Saturday's disappointing FA Cup exit at the hands of Wycombe Wanderers.
Harris made two changes to the team that started at the weekend with George Saville replacing Ben Thompson and Mark Beevers in for Sid Nelson to partner Byron Webster at the heart of the back four. Both Thompson and Nelson dropped to the bench.
With so much at stake it was a surprisingly open game from the start with both teams trading scoring chances evenly throughout.
In the opening quarter of an hour a sweeping move down the left saw Joe Martin's pass chested into the path of Aiden O'Brien, who weaved his way towards goal and sent over a low, inviting delivery that flashed across the face of goal.
It then took a superb flying save by Jordan Archer to fingertip away Dave Worrall's 25-yard piledriver as the ball arrowed towards the top corner on 15 minutes.
That moment of athleticism by Archer invited praise, and it duly arrived as the large bank of 1.500+ Lions fans behind the goal chanted the keeper's name.
Saville had an angled drive blocked before Adam Barrett tested Archer with another long range effort, but this time the keeper collected comfortably low down with 26 minutes on the clock.
It was Archer's opposite number Dan Bentley who was called into action eight minutes later, spreading his body well to thwart Gregory, who had done well to win possession high up the field and knock the ball past Barrett before taking aim at goal.
However The Lions didn't have to wait much longer before breaking the deadlock, with skipper Morison's sweetly struck half-volley at the far post after Carlos Edwards' dinked delivery had been flicked into his path by Gregory.
Although on the balance of play that goal may slightly have come against the run of play, once ahead, The Lions were in no mood to relinquish their precious lead, although there were a few anxious moments along the way.
Early in the second-half a lapse of concentration by Ed Upson handed Worrall an opportunity 16 yards out, but thankfully the Southend man snatched at his shot and the ball flew well over the bar.
Worrall went even closer, just moments after Webster's shot had been hacked off the line following a Millwall corner, but once again the Shrimpers' player was off target with a low effort that whistled past the upright.
Williams was denied a second goal for Millwall when his 55th-minute curling free-kick was expertly palmed away by Bentley.
Unfortunately for Southend the keeper then undid his good work with a howler 10 minutes later, allowing Williams' low grubber from 16 yards to squirm through his hands and over the line, sparking wild celebrations by the Lions players and fans.
Boss Harris introduced Shane Ferguson for goalscorer Morison with 19 minutes remaining. O'Brien moved up front to partner Gregory, Upson switched flanks to the right and the sub slotted in on the left side of midfield.
Southend brought on their second sub of the game almost immediately. Joe Pigott had already entered the fray just before the hour mark but the introduction of Glen Rea saw the home side go for broke, with three at the back in a bid to get a foothold back in the game.
But it was The Lions who so nearly put the tie beyond reach with a third goal nine minutes from time when Saville's drive was beaten away by Bentley, who then reacted smartly to divert Gregory's follow-up for a corner.
With the home side desperately pressing for a glimmer of hope a slip by Webster so nearly handed them an opportunity to reduce the deficit as the game entered the final five minutes.
Sub Pigott nipped in and picked out David Mooney in a central position 14 yards out, he in turn controlled the ball and let fly with a low shot that Archer clawed away at full stretch.
Other than another effort from Worrall that flew wildly over the bar, The Lions comfortably saw out the final few moments plus three minutes of injury-time to move within touching distance of a fourth Wembley appearance in seven years.
Southend United: Bentley, White (Rea, 73mins), Coker, A.Thompson, Worrall, Mooney, Payne, Atkinson, Hurst (Pigott, 58mins), Leonard, Barrett. Subs not used: Prosser, Deegan, O'Neill.