Ashley Nadeson opened the scoring in the first half before Ryan Colclough levelled for the Iron in the second period. However, Reece Grego-Cox restored the lead minutes later.
The hosts weren’t finished there, netting twice in the space of two minutes in injury time to cap off the win at the People's Pension Stadium.
Seven minutes in, Grego-Cox fired over the first meaningful chance in the contest, side-footing narrowly over the bar after receiving Lewis Young’s low ball in from the right.
Junior Brown registered the Iron’s first shot on target, with 19 minutes on the watch. After breaking the offside trap, Jamie Proctor laid the ball off to Matthew Lund on the edge of the area.
The midfielder cut the ball back to Brown, whose first-time effort stung the palms of Glenn Morris.
The game came to life after that. Moments later, Grego-Cox made a run down the left and cut inside before taking aim but the shot lacked conviction and was comfortable for Jake Eastwood.
Proctor and Alex Gilliead linked up well down the right before the latter exchanged passes with James Perch and whipped the ball into the ball which was cleared away by the knee of Nathan Ferguson.
The Iron came knocking again, in the 26th minute. Ryan Colcough’s neat step-over made space to find Brown, making inroads down the left, but the left-back’s cross was blocked.
On the half-hour mark, the Iron won a corner. Colclough’s delivery was won by Yann Songo’o who headed straight at the ‘keeper.
At the other end of the pitch, David Sesay let fly from outside the box, his effort flashing wide of the upright.
Crawley broke the deadlock with 10 minutes remaining of the first period. Ashley Nadeson netted, nodding the rebound over the line to make it 1-0 to the Red Devils after Ferguson saw his long range effort saved by Eastwood.
Ferguson looked for a goal of his own, driving forward before unleashing an effort which rolled just wide.
United could have levelled two minutes later. The Iron were awarded another corner which Proctor got himself on the end of but couldn’t keep his header down.
Eastwood was called into action once again to smother Sesay’s shot which appeared to take a nick off Gilliead.
In added time, Songo’o went close. The hosts failed to clear Colclough’s cross, which eventually fell to the Iron number five 25-yards out but his effort was deflected behind and the Red Devils took a slender lead into the break.
United’s first chance of the second-half fell to Proctor. Perch picked out Gilliead on the right, who crossed for the striker but he couldn’t direct his header on target.
Colclough spurned an opportunity for the Iron with 57 minutes on the clock. Kevin van Veen’s free-kick came back off the wall and dropped for the winger whose half-volley was saved by Morris. He followed up but the ‘keeper was equal to the header.
In the 67th minute, Ferguson had a speculative effort after skipping past the defence but it was watched wide.
Soon after, United created an opening after Lund won the ball back and substitute Lee Novak cut it back to Colclough whose effort took a deflection en-route to goal.
Ryan Tunnicliffe almost made it 2-0 to the hosts, taking aim from outside the area but Eastwood got down well to make the save.
Eastwood made a superb double save in the final 15 minutes, first parrying out Bez Lubala’s effort and then diving off to his left to keep out the rebound.
The ‘keeper’s heroics paid off as Colclough equalised in the 79th minute. Andy Butler headed a free-kick across goal and the winger netted from close range at the back post to make it 1-1.
However, the parity didn’t last long. Crawley re-took the lead two minutes later through Grego-Cox who rippled the net from 25-yards out.
Eastwood was forced into a save with five minutes left to play. The shot-stopper beat Tom Dallison’s effort away from a corner.
The Iron had one final opportunity in the dying stages of the game to earn a replay but Gilliead fired over the bar after pouncing on a loose ball in the final third.
The hosts sealed passage to the Second Round as Ashley Nathaniel-George made it 3-1 in the third minute of stoppage time and two minutes later, Grego-Cox added a fourth, finishing up 4-1.
CRAWLEY: Morris, Young, Dallison, Grego-Cox, Nadesan (Nathaniel-George 70’), Ferguson, Sesay, Tunnicliffe, Bulman, Camara, Lubala.
CRAWLEY UNUSED SUBS: Luyambula, McNerney, Bloomfield, Allarakhia, Sendles-White, Boadu.
IRON: Eastwood, Lund, Songo’o (Novak 65’), Butler, Colclough, van Veen, Perch, Gilliead, McArdle, Brown, Proctor (Eisa 76’).
IRON UNUSED SUBS: Watson, Clarke, Dales, Bedeau, McAtee.
ATTENDANCE: 1,706 (192 away).
