Madden, 27, had been left out in the cold at Scunthorpe but he showed ex-Town boss Graham Alexander just what he is made of when given the freedom to play his way as he teed-up Diagouraga for the opener and side-footed in his own debut goal to fire Fleetwood to another three points.
It was not a case of the luck of the Irish as there was nothing fortuitous about his attacking vision with the forward schooling Anton Ferdinand for both goals and nearly picking out fellow new boy Diagouraga and then rifling home.
Southend did fight back late on but Michael Kightly's deflected effort made things interesting though in this New Year Uwe Rosler's new look side know how to see games off and survived some late scares too rise up to 11th in the league.
Diagouraga had only trained a couple of times with his new team mates after his arrival from Plymouth prior to getting the nod in the midfield three but definitely made his presence known.
That was one of two changes Town make to the side that drew 0-0 with Leciester in the FA Cup third round last Saturday as Diagouraga came in for George Glendon and skipper Nathan Pond came in for the injured Ash Eastham.
It was a less than enthralling first half with the highlight in the press box a batch of warm sausage rolls kindly supplied by the Southend press team on a cold January afternoon.
Fleetwood were the brighter of the two teams in that first 45 with Ash Hunter causing a nuisance on the left flank.
It was Hunter who's probing run lead to him getting felled for the biggest chance of the half. He was tripped just outside the box and then whipped the subsequent set-piece up into the path of Devante Cole who nodded the ball onto the bar.
Fleetwood ended strongly with another piercing counter. Wes Burns lead this one as the half drew to a close but instead of sliding Dempsey in on his left, he overran the ball and the threat was lost. It was an attack that summed up the first 45 as both teams toiled on a chewed up Roots Hall turf.
After a tame opening the game sprang to life in the second half with both Town wing-backs Bell and Coyle blasting efforts wide.
There was a potiential game changing moment just after the 50 minute mark when Diagouraga made a meaty challenge on Demetriou.
It was a tackle that Southend boss Phil Brown was to call a 12 stud challenge and rue the colour of the card, as he believed it warranted a straight red card.
The real pivotal moment came seconds later when Rosler opted to freshen up the Fleetwood front line and send new boy Madden into the mix.
His fresh legs and desire to make a mark after leaving Scunthorpe with a point to prove lead to a man determined to show what he could do given a chance to play in a position that suits him.
Town's counter-attacking brand of football definitely played into his hands and he made an immediate impact picking up an Ash Hunter flick to loft the ball into the path of fellow new signing Diagouraga who fired home.
Then the forward got his own name on the team sheet, Dempsey's intuitive flick saw him sprint towards goal and he outclassed a static Ferdinand to double Town's lead.
McAleny joined the party late on as Town added more firepower onto the pitch in a revamped side that is now ready to rise up the table.
Town though could not get a third despite a probing Hunter cross from the right evading everyone.
And Rosler's men nearly paid the price for not increasing their lead as a fluke late Kightly goal set-up a tense finish. His deflected effort looped over Neal with the hosts throwing everything and even keeper Max Oxley, though not the kitchen sink as they tried to find a leveller to stop a rot that has now seen them lose eight of their last nine.
Town have grown in stature in recent weeks and have learned how to see off games as they withstood a late Southend fight back to clinch all three points and with their fifth straight away league game in all competitions.
Fleetwood: Neal, Coyle (Jones, 84) Pond, Bolger, Bell, Diagouraga, Schwabl, Dempsey, Hunter, Cole (McAleny, 75), Burns (Madden, 56) Subs not used: Cairns, Hiwula, Grant, Sowerby.
Southend: Oxley, Coker, Ferdinand (Fortune, 65), Turner (Hendrie, 24) Kightly, Timlin, Cox (Wabo, 69), McLaughlin, McGlashan, Demetriou, White. Subs not used: Bishop, Ba, Kyprianou, Bwomono.
Referee: Anthony Coggins