City, playing incisive, passing football on the break, never looked back from Forrester's first treble for the club, notched inside 15 remarkable minutes by the half hour, to record a crushing derby day victory.
And when the Stags, who always looked dangerous in attack themselves, did threaten a comeback, Lincoln found another hero in keeper Alan Marriott.
He pulled off several vital close-range saves in a thrilling first half and capped his performance with a fine penalty save to stop Stags' hopes of a fightback on the stroke of half-time.
The Stags were almost given the perfect start when a neat exchange of passes by Gareth Jelleyman and Richie Barker put Michael Boulding clear, but his cross from inside the area was easily cut out by Paul Morgan.
Boulding then wriggled past two challenges inside the area and set up Barker, but the striker's shot on the turn was well held at the foot of the post by the Marriott.
At the other end a diving block by Jake Buxton kept out Scott Kerr's piledriver after a slip from Jonathan D'Laryea on one of several incisive Lincoln breakaways.
But there was no let-off in the 15th minute. A clumsy challenge from Alex Baptiste brought down Lee Frecklington and Forrester tucked away the penalty, just past the despairing fingertips of Carl Muggleton.
Boulding was denied an immediate equaliser by Marriott's brave stop at the striker's feet and then the keeper held on to Simon Brown's 25-yard speculative drive.
Those saves proved vital in the 23rd minute when Forrester doubled City's lead with a superb solo strike on yet another dangerous quick break.
The striker raced unchallenged across the area, then waited calmly before turning to curl a high class finish into the top corner from 12 yards.
Seven minutes later the striker completed his hat-trick. A sliced clearance by Baptiste fell to Ryan Amoo on the right edge of the area, who crossed to the far post, where Forrester's fierce header was acrobatically clawed out by the keeper, only for Forrester to smash in the rebound.
Mansfield worked hard to get back into the game, but their hopes were dashed just two minutes from half-time by three superb saves from Marriott.
First the keeper twice denied Brown inside the six yard area and then when Lee Beevers tripped Boulding to concede a penalty, Marriott dived low to his left to beat out Barker's penalty.
Yet another penalty, the third of the game, arrived in the 51st minute. Johnny Mullins tripped Mayo and Forrester slammed in his fourth goal and second spot kick - down the middle.
Substitute Danny Reet netted the Stags' late consolation, finally beating Marriott with a low 68th minute drive moments after another good save from the keeper had thwarted Brown's powerful angled shot.
Morgan then nodded Boulding's 76th minute header off the line, when Barker cleverly flicked Dawson's deep cross back across goal, and Marriott beat out Dawson's piledriver to end the home side's hopes of an unlikely late fightback.
But in injury time the Stags did get a second goal, Barker finally converting from the penalty spot after his earlier miss, when Frecklington handled a Brown cross.