Josh Low's fifth goal of the season settled the destiny of the points in a physical battle at Sixfields and kept alive the Cobblers' hopes of making the end of season shootout.
Lincoln had won four of their previous five games and needed just a point to be certain of making the play-offs. Next Saturday they entertain Darlington who currently occupy the final play-off place which the Cobblers are casting their eyes upon.
The crucial moment arrived with 21 minutes left when Charlie Hearn's corner brought a volley from Scott McGleish which Low managed to touch home just a yard from the goal-line.
Most of the goalmouth action was sandwiched into the final 20 minutes when the Cobblers had several quality chances to add to their lead.
McGleish shot tamely just minutes after the goal, while also curling a shot just wide of the far post.
And in the closing stages, both Hearn and substitute Marc Richards had chances to relieve the tension.
Lincoln had fewer openings, although they almost pinched a point in injury time when leading scorer Simon Yeo saw his shot rebound back into play off a post.
The first half had its share of ugly confrontations, which meant there was little happening in front of goal.
McGleish had the best of the home chances with a header which was tipped over the crossbar by Alan Marriott, while at the other end Gary Taylor-Fletcher should have done better with a glancing header from Kevin Sandwith's free-kick, the ball drifting harmlessly wide.
Taylor-Fletcher could have put Lincoln ahead five minutes before the break when he timed his run to meet a Sandwith corner to perfection, but the ball flew harmlessly over the crossbar.