Goals from Sean Longstaff and Ivan Toney put the Magpies in command early on at The Northern Echo Arena, but a trialist pulled one back for Darlo before the break after Toney saw his penalty saved by Peter Jameson.
Peter Beardsley opted to field an entirely different team in the second half against Martin Gray's men, but the scoreline remained the same as the young Magpies got their preparations for the 2017/18 campaign off to the perfect start.
After debutant James Caton had twice tried his luck and David Syers had seen his shot tipped over by Nathan Harker, the hosts fell behind despite their encouraging start. Callum Smith broke free on the left, sending a through ball into the path of Longstaff, who coolly found the bottom left-hand corner.
Barely two minutes later it was two, as Toney - who enjoyed loan spells at Shrewsbury Town and Scunthorpe United last season - turned his man in the area and fired a left-footed strike past Jameson into the roof of the net.
And it could have been three midway through the half. Jameson's misplaced clearance gave Toney the chance to go one-on-one, but when the Darlo keeper brought him down, United were awarded a penalty. Toney stepped up, but Jameson redeemed himself, parrying the Magpies' frontman's effort to safety.
Martin Gray's men rallied, though, and reduced the deficit after 26 minutes, thanks to their pair of trialists. One led the hosts' breakaway down the right, and it was his incisive pass that gave the second trialist the opportunity to set himself and dispatch a low shot beyond the reach of Harker.
After Toney struck the base of the post and the Darlo trialist saw a close-range equaliser chalked off for offside, Beardsley changed his entire team at half time, bringing 11 players off the bench.
And it was one of the replacements who went close with the first chance of the second period, as Owen Gallacher glanced Ben Kitchen's inswinging cross past the post.
United continued to apply pressure, with Luke Charman playing in Juanito on the left side of the penalty area. The Spaniard's shot, though, didn't trouble Jameson as it flew over the crossbar.
Though the contest perhaps wasn't quite as free-flowing as it had been in the first half, there were still sporadic opportunities for the Magpies to extend their lead. Another one of those fell to Liam Smith, but Jameson did just about enough to smother the shot and allow his defender to clear the danger.
Darlington had another goal chalked off with just under quarter of an hour remaining as Jordan Richards' free kick found the bottom corner, but saw the leveller ruled out. Despite that late pressure from the hosts, United held on to claim the win in their first outing of the summer.
Newcastle United Under-23s (first half): Nathan Harker, Callum Williams, Kyle Cameron, Jack Hunter, Curtis Good, Stefan O'Connor, Yasin Ben El-Mhanni, Dan Barlaser, Ivan Toney, Sean Longstaff, Callum Smith.
Newcastle United Under-23s (second half): Otto Huuhtanen, Michael Newberry, Josef Yarney, Stuart Findlay, Macaulay Gillesphey, Ben Kitchen, Dan Ward, Liam Smith, Juanito, Owen Gallacher, Luke Charman.
