The Black Cats romped into an early two-goal lead through David Connolly and Tobias Hysen but had to wait until the later stages and a double strike from Stern John to finally kill off their opponents.
The Wearsiders survived an early scare, the lively Jamal Campbell-Ryce creating a chance for himself only to shoot wide, before storming into a two-goal lead inside 13 minutes.
Dean Whitehead played the ball down the right and a hesitant Peter Clarke failed to cut out the danger as he allowed Carlos Edwards to head for the byline.
Edwards delivered an astute pass for Connolly to score his ninth goal of the season from close range.
Whitehead was again in the thick of the action in the 11th minute, crossing for John to send a diving header against the upright with goalkeeper Darryl Flahavan well beaten.
The home side were not to be denied, however, and two minutes later deservedly increased their lead. Connolly turned provider to release Hysen who cleverly beat Lewis Hunt before sliding the ball past Flahavan.
A subdued Freddy Eastwood fired weakly wide but it was a rare attacking move by the visitors who looked set to be brushed aside with ease as the Black Cats took complete control.
Whitehead and John had shots blocked, Edwards drilled a low centre across the face of goal to cause further panic and then a header from John was superbly tipped over by Flahavan.
Steve Hammell had a free-kick saved and only a last-ditch clearance by Jonny Evans prevented Lee Bradbury from pulling a goal back after he had slipped the ball past Marton Fulop.
Sunderland were denied a clear-cut penalty when Connolly went down when challenged from behind and, from the resultant melee, the home side were awarded a free-kick outside the area.
All hell broke loose with three players booked and, in a last throw of the dice, Southend boss Andy Tilson sent on Richie Foran, Matt Harold and Franck Moussa in a forlorn triple substitution.
The Black Cats then upped the pace to score twice in as many minutes late on.
Whitehead crashed the ball against the post in the 76th minute and John pounced to score from the rebound and, two minutes later, Liam Miller centred for John to sweep the ball home at the far post.
Southend's misery was compounded when Campbell-Ryce was carried off in the 85th minute to leave the Shrimpers to finish the match with only ten men.