The midfielder's tidy finish just before the interval was all that separated the sides at a rain-soaked St. James' Park as Eddie Howe's men suffered their first Champions League defeat since returning to the competition after a 20-year absence.
It could have been different had Dortmund's goalkeeper Gregory Kobel not produced a crucial save to keep out Callum Wilson in the second period, while both Wilson and Anthony Gordon both struck the crossbar in the closing minutes, but in truth Newcastle just didn't create quite enough to break down a resolute yellow wall.
The first ten minutes were played at breakneck speed, the slickness of the turf after some heavy rain contributing to some free-flowing attacks. The dangerous Donyell Malen forced Nick Pope into a smart save and the similarly lively Gordon tested Kobel early on, but it was the Newcastle stopper's interventions which caught the eye; soon after Pope produced a fine double save to repel Malen and then Niklas Fullkrug's follow-up attempt.
It was attack followed by counter-attack and another breakaway saw Alexander Isak slide in Gordon, whose curler was beaten away by Kobel, but the Magpies' Swedish forward succumbed to injury soon after. With Isak off the pitch and Wilson waiting to replace him, Malen whipped another shot over the top.
The German side went into the break with the lead, though. Isak's withdrawal seemed to stem some of the hosts' flow in the final third but Dortmund still looked a threat. Centre back Nico Schlotterbeck pinched possession from Gordon in his own half and continued his run after feeding Malen. The Dutchman returned the favour as Schlotterbeck overlapped down the left and his cutback was perfect for Nmecha, who sidefooted home.
The otherwise quiet Marco Reus blasted a speculative effort over after the restart but before the hour, Howe's charges carved out a golden opportunity to level the scores. It came thanks to a thumping sliding challenge from Fabian Schär, who beat Fullkrug to the ball in a 50/50. Wilson played Gordon in down the left and the winger's centre found United's number nine, who took a touch and looked primed to equalise, but Kobel stood up to the shot and blocked it with his feet.
Joe Willock's return from injury after six months - in place of fellow substitute Jacob Murphy, who had to come off injured after his own introduction - gave the Newcastle faithful cause for optimism during the final quarter, but real clear-cut chances were still at a premium. With less than five minutes remaining, the Magpies went agonisingly close to a leveller when Wilson glanced a Matt Targett free kick goalwards, but the ball cannoned back off the crossbar.
And moments later, in stoppage time, Gordon swivelled and his shot took a hefty deflection before looping over Kobel and striking the woodwork once more, with Dortmund moving swiftly to scramble it away and maintain their grip on the three points.
