With rain lashing down it took the young Imps just 90 seconds to open the scoring when a fine move down the right ended up with Conner Robinson skipping past his marker before crossing into the danger area. There to meet the ball at the back post was left-back Will Rawdon who finished nearly past Kieron Coupe in the Mansfield goal.
Shortly after the restart Robinson fired wide and Aaron Rice had a shot blocked by Coupe's legs before the former skinned his man again before winning the first corner of the game. Two more followed and the pressure eventually paid off in the 12th minute when Jordan Thomas doubled the scores; the striker producing a neat finish after he was released down the middle by Matt Bowles.
Mansfield's first effort saw Joe Flint drag a low effort well wide whilst at the other end Thomas cut in from the left before seeing his low shot held by Coupe at the second attempt.
Midway through the half Rob Taylor fired a free-kick over after Robinson was tripped on the edge of the box by namesake Dean and shortly afterwards the City striker had a header saved by Coupe as he went in search of a third.
Coupe then produced a superb one-handed save from Robinson after he pressed again and pulled off a similar stop to keep out Thomas. He was powerless, however, when the resultant corner from the latter save was headed into the roof of the net by Thomas, who met Taylor's inswinger perfectly.
Thomas, searching for his hat-trick, was denied by Coupe's left boot as the Imps continued to dominate proceedings. An eighth corner was then cleared by a relieved Stags' defence as the half-time whistle approached.
HT: Imps U18s 3-0 Mansfield U18s
Mansfield had their best spell of the game in the opening five minutes of the second half with Ollie Pannell commanding his box well to claim their first corner. A city corner followed and when the ball was played back to taker Taylor he saw his chipped effort.
It wasn't long before the Imps were pushing forward again though and when Robinson got the better of his marker, he duly bore down on Coupe before neatly slotting home.
Four became five shortly before the hour mark when Thomas picked himself up after being bundled over in the box to send Coupe the wrong way from the spot to compete his second successive cup hat-trick.
Rob Smith came on for Kegan Everington for City's first change and shortly afterwards Pannell was called into action low to deny Stags' substitute Remi Davis, who then headed a Marsden cross onto the crossbar with Pannell beaten.
The Imps' response to this scare saw them push forward and after Robinson saw his 69th minute effort blocked by Coupe, there was Thomas to blast into the roof of the net from eight yards out. Play then switched to the other end after Robinson fired into the side netting with Flint firing past Pannell from a narrowing angle to make the score 6-1.
Cotton then had an effort cleared off the line but it wasn't long before the Imps were on the attack again with Matt Cotton, on for four-goal Thomas, doing well down the left before pulling the ball back to Matt Bowles who promptly curled home a beauty from 25 yards.
With the game heading towards its conclusion, some great play from Cotton created a half-chance for Rice who fired over before full-back Rawdon took a bow when he fired home spectacularly with the outside of his left boot from distance.
FT: Imps U18s 8-1 Mansfield U18s
City U18: Ollie Pannell, Nick Green, Will Rawdon, Kegan Everington, Luke Weaver, Ryan Leggett, Rob Taylor, Matt Bowles, Jordan Thomas, Conner Robinson, Aaron Rice.
subs: Kieran Walker, Matt Cotton (for Taylor 72), Charlie Brownlie, Alex Simmons (for Thomas 77), Rob Smith (for Everington 62).
goals: Rawdon (2, 90), Thomas (12, 30, 59 pen, 69), Robinson (52), Bowles (78)
Mansfield U18: Kieron Coupe, Jordan Jones, Liam Marsden, Jason Gregory, Billy Simpson, Jack Thomas, Matty Jones, Dean Robinson, Danny Barke, Joe Flint, Jack Hawkins.
subs: Tom Clarke, Remi Davis (for Barke 53), Ben Birch (for Gregory 53), Callum Pursglove, Lewis Chambers (for Robinson 82).
goal: Flint (74)
Referee: Benn Cass