Andy Brown's goal secured Nuneaton promotion from the Blue Square Bet North at the expense of Gainsborough.
Trinity made a bright start to the play-off final, Ryan Kendall's shot was deflected wide while Darryn Stamp's chipped effort also missed the target.
But it was striker Brown who broke the deadlock as he headed home from a floating free-kick to give Boro the lead on 16 minutes.
Kendall continued to trouble the Nuneaton defence but they held on.
It is the third time Boro have won promotion in the last four seasons and will play in non-league's top-flight next term.
VIEWS FROM THE DRESSING ROOM
Nuneaton striker Andy Brown told BBC Radio Coventry and Warwickshire:
"Sometimes scoring a goal early is a little bit of a curse, but credit to the boys, especially the defenders, they had to deal with a lot of pressure for something like 75 minutes of the game, but they never really looked like scoring.
"They didn't really look to play - it was a hard pitch to play on as you could see by the quality of the game - they looked to hit it forward, and that is hard for the defenders, so fair play to our defenders for standing up to it.
"To come this far on a Sunday and get that kind of support to almost make it a home game, I'm sure that played a part."