Eddie Howe's side were indebted to goalkeeper Nick Pope, who pulled off a string of crucial saves in the first half in particular to keep his team in the contest at a sweltering San Siro.
But Newcastle were dogged when they needed to be and disciplined throughout as they turned in a fine defensive display to make sure they would take a point with them back to Tyneside on their long-awaited return to the continent's elite battleground.
The hosts were comfortably on top early on at the famous old stadium, and but for Pope they would have led at half time. Rafael Leão sounded the first warning from range but his weak drive proved just a warm-up for the England stopper, who then had to repel a Tommaso Pobega strike from range before parrying Samuel Chukwueze's back post header.
The wily Olivier Giroud saw Pope throw himself in the way of his instinctive close range flick as chances kept coming for Milan. Rade Krunić's shot was pushed over by Pope, who then pulled off another save from Théo Hernandez's header from the resulting corner, diverting it over the bar.
United had to endure the first quarter - Pope made six saves in all during that hectic start - but was spared having to make another one when Giroud fired Hernandez's low centre past the upright on the half hour. Leão then slalomed his way into the box and looked set to pull the trigger before losing his footing, allowing Newcastle a chance to reset, with Jacob Murphy retreating to clear Pobega's effort off the line.
Howe's men failed to test Rossoneri goalkeeper Mike Maignan before the break but Sandro Tonali, on his return to the club and stadium he departed in the summer, looked to change that just after the restart, seeing his curling shot charged down on the edge of the box.
It was a solid enough start to the second period from the visitors, with Murphy sailing one way too high soon after as the Magpies pushed up the pitch. There were still gaps which Milan looked to exploit, though; one such opening fell the way of substitute Alessandro Florenzi, who dragged wide after finding himself in space behind Newcastle's midfield trio.
Howe introduced Miguel Almirón and Callum Wilson for the final half hour as they sought a greater foothold in the Milan half, with one of the hosts' replacements Tijani Reijnders reminding United of their threat with a low shot that Pope held onto.
The hosts' best chance of the closing stages fell to Leão, who ghosted into the box to meet Florenzi's enticing cross but glanced it just over. Some terrific last-gasp defending from United, who threw bodies in front of the ball again and again to frustrate the Italian outfit, coupled with another Pope save from Giroud kept the scoreline level.
And deep into added time, Newcastle broke, with the ball falling to Sean Longstaff on the edge of the box. His shot was fierce and looked to be flying in, but substitute stopper Marco Sportiello produced a superb reaction save to tip it over the bar and deny those in black and white an unforgettable ending to an absorbing evening.
