With the game looking likely to finish all-square, Nimmo beat the Rothwell offside trap, latched on to a perfectly-weighted 30-yard Paul Goodhand through ball and calmly slotted home the match-winner with just seconds left on the clock.
And while the Town striker was taking the credit, the defence-splitting pass capped a fine performance from Goodhand, who finished with two assists and a goal.
The Poachers began strongly, Miles Mason going within inches of opening the scoring with just three minutes on the clock.
But Mason did find the back of the net in the fifth minute.
Goodhand shielded the ball on the edge of the visitors' area before teeing up the Town striker, who finished low.
The Poachers dominated the early exchanges against their Midland Division One opponents, but were stunned to go in level at the break, Rothwell equalising moments before half time with a simple header at the back post.
Six minutes after the re-start Goodhand put his side back in front from the spot, the penalty awarded for hand ball.
With Gary Bull given the night off, he stepped up and fired the ball fiercely into the net.
But Rothwell pulled level again two minutes later with a strike from distance.
It seemed the away team were the more likely to snatch a winner as Boston's second-half keeper, Nick Maddison, had to pull off two fine saves, diverting a 25-yard drive and tipping a dipper over his bar.
But it was the Poachers who had the last laugh thanks to a bit of Goodhand magic and the cool-headed Nimmo.
TOWN: from – McAllister, Lambley, Pinner, Foster, Baker, Collins, Jeffrey, Goodhand, Mason, Nimmo, Lovas, Brown, Smith, Ogden, Price, Ainsley, Courtley.