The game finished 1-1 after extra time and Grimsby substitute Jon-Paul Pittman then fired his spot-kick over the bar, allowing Mansell to seal a 5-3 shoot-out success for manager Darrell Clarke’s side.
The Mariners had gone ahead after less than two minutes at Wembley when Lenell John-Lewis’ scrappy finish beat keeper Will Puddy.
But the Gas equalised around the half-hour mark through forward Ellis Harrison,who slammed the ball home in front of a record attendance for a Conference final of 47,029.
Clarke took the bold move of replacing goalkeeper Puddy with Steve Mildenhall at the end of extra time but the former Mariners stopper was not needed, as Pittman blazed his penalty over.
Grimsby took an early lead in somewhat fortuitous fashion, the ball bouncing into the net off John-Lewis’ face after his initial shot had been saved by Puddy.
John-Lewis went on to have another effort saved by the Rovers goalkeeper, who then appeared slightly lucky to receive just a yellow card when he came out of his box and handled an Ollie Palmer shot.
Nathan Arnold then went close for Grimsby with a curler from 25 yards out that was just wide, but it was Rovers who got the next goal, levelling in the 29th-minute.
Aristote Nsiala sliced a clearance over his own bar and, from the resulting Jake Gosling corner, Grimsby failed to clear and Harrison smashed the ball into the net from close range.
The momentum was suddenly with Rovers and they thought they had a penalty when Matty Taylor rounded goalkeeper James McKeown and went to ground after capitalising on a mistake by Shaun Pearson, only for the referee to blow for a dive.
The Pirates then had the first chance of the second half as Taylor shot over after good work by Gosling, but the opportunities then began to dry up, with both sides desperate not to make a mistake.
Grimsby sub Danny Parslow saw a cross flash across the Rovers six-yard box and Mansell sliced a shot over at the other end, late on in normal time.
Extra-time was every bit as cagey as the second half, with Chris Lines’ looping header for Rovers and Pittman’s cleared cut-back the only chances of note.
Penalties duly arrived and Lines, Taylor and Lee Brown scored for Rovers, Craig Disley and John-Lewis responding for Grimsby.
However, Pittman then missed and, after Balanta and Craig Clay converted their spot-kicks, Mansell confidently found the net to send Bristol Rovers back into the Football League at the first attempt.