A bumper crowd saw Gainsborough secure their National League North status, despite losing 1-0, as results elsewhere went in their favour.
Trinity’s fellow relegation threatened side Brackley had to win away at Curzon Ashton, but they could only draw in Greater Manchester.
It was an afternoon of a whole range of emotions at the Martin and Co Arena, where a Jerome Wright drive just after the break won it for FC United.
It had looked like only two points would separate the Blues and Brackley going into the last round of fixtures next Saturday, with Brackley a goal up on Curzon with seven minutes to play.
Other sides in and around Gainsborough were winning too to make matters worse.
Fortunately it all worked out for the Blues, overall, even if in their own match they paid the price for not putting the ball in the net.
Gainsborough boss Dom Roma made one change to the side that drew with Lowestoft as Simon Russell returned to the starting lineup after a hip flexor injury in place of Marc Newsham as Roma started with a 4-5-1 formation.
Trinity started a little nervously and conceded a corner to the visitors within the opening thirty seconds.
Jan Budtz claimed with ease and United threatened little after that for the remainder of the half. The longer the half went on, the Blues began to dominate possession and territory.
In the 18th minute Nathan Jarman fired the first warning shot, running onto a through ball over the defence and the attempted lob went straight at the keeper.
Two minutes later Jarman was up ended on the edge of the box and drilled the resultant free kick on target.
Trinity were playing some slick football at this point and after switch of flanks, Jamie Yates curled a shot from the edge of the box at Dylan Forth in the United goal.
Yates was at the centre of attention five minutes later when he was brought down in the box, stepping inside his man and looking likely to shoot had he been given chance.
The referee waved play on despite appeals from around the ground.
Three minutes prior to the interval, Jarman came millimetres away from notching in a fourth straight appearance.
The former Ferriby man stooped to meet a low Stephen Brogan corner, beat the keeper but the defender on the line swatted it away.
The good work Trinity had done in the first half was soon cancelled out, as FC United took the lead in the 53rd minute.
Jerome Wright was the man to score, driving into the box and striking across goal after expertly controlling a cross field pass.
A minute later Wright could have doubled the United lead when his dipping volley thumped the crossbar from the edge of the box.
On the hour, the Blues should have been given a chance from the penalty spot. Jarman held off and turned his marker, shaped to shoot and was tripped, with the referee playing on much to his disgust.
The Blues added Newsham and Darryn Stamp to their front line to grab a goal, leaving plenty of space in the midfield.
It was the former who threatened to score, hitting the side net with a volley and minutes later forced a low save out of Forth.
United had a late chance to extend their lead when Tom Greaves was clean through but Budtz pulled off an excellent save to tip it wide, with the referee insisting that it went straight out for a goal kick.
Luckily results elsewhere ensured the Blues would have a pressure free day at Bradford on Saturday with a late Curzon Ashton goal making them untouchable by Brackley in the last relegation spot.
Gainsborough Trinity: Budtz, Roma, Lacey, D’Laryea (Wilson 86), Quinn, Picton, Russell, Yates (Stamp 77), Brogan, Jarman, Davis (Newsham 62). Subs (not used): Binns, Hedge.
FC United: Forth, Tonge, Chantler, Brown (Winters 71), Eckersley, Ashworth, Wolfenden (Greaves 45), Sheridan, Johnson (Daniels 71), Wright, Thomson. Subs (not used): Walwyn, Carnell.
Goalscorers: Wright 53.
Referee: Mr R Wooton. Assistants: Mr P Henger, Mr P Wright.