Cheltenham striker Steve Guinan scored the winner to earn his side promotion after victory in the League Two play-off final against Grimsby.
There was a touch of fortune around Guinan's goal
Grimsby were denied a penalty when Andy Parkinson was brought down and their misery was compounded when Guinan's angled cross crept in at the far post.
Grant McCann had a chance to extend the Robins' lead but missed a spot-kick and almost paid the price late on.
Gary Jones had an excellent chance for Grimsby only to see his header saved.
An uneven pitch made for difficult conditions at the Millennium Stadium but Cheltenham should still have made more of two glorious chances in the first 10 minutes.
Robins striker Steven Gillespie beat lethargic Grimsby central defender Rob Jones to McCann's long ball and left himself with just keeper Steve Mildenhall to beat after nodding goalwards.
But Gillespie's second touch let him down and allowed the Grimsby stopper to smother a shot at his feet.
Gillespie missed an early chance for Cheltenham
Almost immediately, Guinan pounced on a tame Jones backheader but went for a spectacular half-volley when he had time to control the ball and pick his spot.
Both teams suffered a setback when a clash of head between Mariners forward Michael Reddy and Cheltenham's Craig Armstrong resulted in both players being substituted.
Grimsby striker Parkinson stood out in a scrappy affair and he should have earned his side a penalty after he was brought down by Gavin Caines.
The referee waved away the Mariners' appeals but they ended the first half strongly, with Cohen having a shot palmed away while Parkinson curled an effort just over.
Caines had a header easily saved after the break as the match lacked sparkle and any element of urgency.
But that did not last for long as Guinan's cross crept in at the far post to give the Robins the lead and bring the match to life.
Cheltenham were awarded a penalty for Curtis Woodhouse's challenge on McCann only for the Robins midfielder to pick himself up and see his spot-kick saved by Mildenhall.
The Mariners stopper was again quickly called upon as a flurry of action ended with him blocking a point-blank Brian Wilson header after McCann's floated shot had come back off the crossbar.
At the other end, Cheltenham keeper Shane Higgs saved a close-range header from Jones as he helped his side hold on for a precious win.
Grimsby: Mildenhall, Croft (Futcher 75), Robert Jones, Whittle, Newey, Mendes (Goodfellow 83), Woodhouse, Bolland, Parkinson, Reddy (Cohen 27), Gary Jones.
Subs Not Used: Kamudimba, Toner.
Cheltenham: Higgs, Gill, Caines, Duff, Armstrong (Bell 27), Vincent (Spencer 77), Finnigan, McCann, Wilson, Gillespie (Odejayi 62), Guinan.
Subs Not Used: Brown, Bird.
Goals: Guinan 63.
Ref: P Taylor (Hertfordshire).