Two first half goals from the home side ultimately did the damage, despite the visitors getting themselves back into the game through a Paddy Madden penalty.
United went into the game unchanged from the 2-0 away victory at Millwall. Luke Daniels started in goal, and there was a back four of Jordan Clarke, Murray Wallace, David Mirfin and Conor Townsend. Jamie Ness, Stephen Dawson and Neal Bishop started in a midfield three for the visitors, with Madden, Tom Hopper and Gary McSheffrey in attack.
There was an early warning for the visitors, three minutes into the game, when debutant Shola Ameobi drew a smart save from Daniels. The former Newcastle man found a yard of space at the near post, before hooking the ball goalwards. However, the United goalkeeper got across well to palm his effort out for a corner.
Right from the off though, the game had an ebb and flow, with Madden soon going close, after Clarke had found him in space with a cross from the right.
Shortly after Chris Maxwell had foiled the forward, Stephen Pressley's side edged ahead, with a neat finish from Ashley Hunter. The forward did well to initially break the offside trap, and then loop the ball over Daniels, with a first time effort.
McSheffrey almost responded immediately for the Iron, with a free kick from the edge of the area but, despite getting his effort on target, Maxwell dealt with the shot without too much difficulty.
Things soon got worse for the visitors, when the home side doubled their advantage on the half-an-hour mark. Former United man Bobby Grant rose the highest in box, before glancing the ball into the far corner of Daniels' goal.
The Iron pushed for a route back into the game before the break, but chances were hard to come by, and the visitors went in trailing by a two goal margin.
United started the second period on the front foot and, five minutes into the half, they went close with an effort from Hopper. After good initial play from Bishop on the right flank, Hopper received the ball on the turn and tested Maxwell with a low drive.
The visitors continued to apply the pressure, with Wallace the next to try his luck with a close range volley. The Scot would be frustrated though, as the Cod Army defence managed to block his strike.
With twenty minutes to play, United's persistence paid off when Hopper was fouled in the box by Stephen Jordan, leaving referee Ross Joyce with no choice but to point to the spot. From the resulting kick, Madden stepped up and sent Maxwell the wrong way, to halve the deficit.
The visitors' tails were evidently up, and they continued to create openings. Substitute Greg Luer nearly brought the Iron level with an ambitious left footed volley from the edge of the area. The Hull loanee struck the ball sweetly, but his effort flashed just past the post.
Luer had sight of goal again moments later, when he fired high and wide after Ness pulled the ball back to him, just inside the box.
The forward had his third effort in quick succession, when he peeled off at the far post and headed Townsend's cross into the arms of Maxwell, who did well to hang on, as Madden was closing in.
Pressley's team could have have been out of sight, if not for Daniels, who pulled off a fine double save five minutes from time to deny Jimmy Ryan
, who shot twice from close range.
As full time ticked ever closer, United loaded bodies into the box, but they couldn't find the elusive equaliser.
Maxwell, McLaughlin (Forbes, 45+1), Jordan, Jonsson, Bell, Ryan, Ameobi, Grant (Nirennold, 81), Sarcevic, Hunter (Burns, 75), Nilsson.
Substitutes not used:
Ball, Harris, Gogic, Cole.
Daniels, Clarke, Wallace, Mirfin, Townsend, Ness, Dawson (King, 21), Bishop, Madden, Hopper (Luer, 72), McSheffrey (O'Brien, 57).
Substitutes not used:
Anyon, Laird, Wiseman, Wootton.
3,098 (335 away).