Chesterfield had to labour for 47 minutes with ten men after Dion Donohue was dismissed and conceded three second half goals.
Will Atkinson burst into the box on 15 minutes and with the ball loose, Thorsten Stuckmann rushed out to try and nick the ball off him but he was late and brought down the winger. The penalty was awarded and Cox stepped up and sent the German 'keeper the wrong way to give the visitors the lead.
The first half carried on in similar fashion with Chesterfield’s first attempt on goal coming in the 35th minute when Dan Gardner’s speculative drive from 30 yards went comfortably wide.
Southend had a goal ruled out for offside a minute later after Cox headed in a Jason Demetriou cross.
It got worse for Chesterfield as Donohue was sent off for a high challenge on Wordsworth just before the break.
The 10 men began brightly in the second half as they went in search of an equaliser. Gardner played Paul McGinn in behind and the right-back played a dangerous ball across the six yard box that eluded everyone.
It proved costly as the Shrimpers went down the other end and doubled their lead when White played the ball across to Wordsworth who swept the ball past Stuckmann in the 58th minute.
The visitors put the game to bed seven minutes later when Nile Ranger teed up Simon Cox after a scramble in the Chesterfield area.
It was 4-0 after 72 minutes when Wordsworth lofted the ball into substitute Marc Antoine Fortune who slotted the ball past Stuckmann.
The hosts came close to pulling one back when Rai Simons almost found Kristian Dennis with a low cross but despite the strikers best efforts, he couldn’t meet the ball and it went behind for goal kick.
Stephen McLaughlin almost scored a sensational goal when he saw Stuckmann off his line and went for the spectacular from 45 yards but his effort dropped onto the roof off the net.
Chesterfield: Stuckmann, McGinn, Jones, Hird (C), Gardner (Simons), Dimaio, Donohue, Dennis (Ebanks-Blake), Anderson, Maguire, Rowley (Grimshaw).
Subs not used: Fulton (gk), Angel, Mitchell, Wakefield.
Southend United: Walton, White, Wordsworth, Thompson, Timlin, Cox (Robinson), McLaughlin, Atkinson (Hines), Demetriou, Ferdinand (C), Ranger (Fortune).
Subs not used: Bexon (gk), O'Neill, Amos, McGlashan.