Leon Cort and player-manager Rob Newman struck in the space of nine minutes to set Southend on their way to a 2-0 victory over United.
Cort opened the scoring after 16 minutes with a shot which flew into the roof of the net and moments later Newman, playing as a makeshift striker, doubled the advantage with a left-footed volley.
Ian Selley played a perfect ball to Newman and he hammered home a superb volley.
Lee Hodges was twice denied by Southend goalkeeper Daniel Gay and Alex Calvo-Garcia saw a header hit the bar before Steven Clark was dismissed in the last minute after headbutting defender Nathan Stanton.
Martin Carruthers made his first appearance of the season, coming on for loanee Mike Jeffrey in the second half.
Action:
17 min - Goal, Leon Cort, (Southend)
26 min - Goal, Robert Newman, (Southend)
45 min - Substitution, Beagrie on for Wilcox, (Scunthorpe)
62 min - Substitution, Carruthers on for Jeffrey, (Scunthorpe)
69 min - Booking, Carl Bradshaw (Scunthorpe), Unsporting behaviour
82 min - Booking, Lee Hodges (Scunthorpe), Dissent
83 min - Substitution, McCombe on for Ridley, (Scunthorpe)
84 min - Booking, Barrington Belgrave (Southend), Dissent
90 min - Booking, Mark Jackson (Scunthorpe), Unsporting behaviour
90 min - Sent off, Steven Clark (Southend), Violent Conduct