Joe Willock's late goal sent a relieved Newcastle into the fourth round of the Carabao Cup as the Premier League side scraped to a 1-0 victory at Sky Bet Championship outfit Millwall.
Just before going ahead, the Magpies had been given a fright when a shot from Tairyk Arconte hit a post, with the hosts at that point threatening to upset the competition's winners of two seasons ago.
The Lions brought on 15-year-old midfielder Regan Anderson, who became Millwall's second youngest player, in the time that remained, but Newcastle held firm in what was a hard-fought tie.
Jacob Murphy bent a shot harmlessly over in the early exchanges in what was the first meeting between the two teams since 1993.
The first time Millwall's debutant goalkeeper Filip Marschall was called into action was in the 15th minute when he parried Willock's strike into the air, with the ball dropping to left-back Elkan Baggott rather than the lurking Yoane Wissa.
The visitors dominated the early proceedings and came close again when Sven Botman headed Murphy's corner narrowly over at the far post.
A rare foray out of their own half by the hosts led to their first half-chance as Lyndon Dykes' hooked effort from Baggott's cross was gathered easily by Lukas Hornicek.
The Lions were providing a threat from set-pieces, with a good delivery from a free-kick by Camiel Neghli being met by a flying header from Caleb Taylor that went just wide.
Newcastle then almost had a stroke of luck at the other end as Fabian Schar's long-range effort took a big deflection and ruffled the side-netting, with Marschall stranded.
It remained deadlocked at the break and Millwall began the second half brightly with Jenson Metcalfe smashing a long-range effort just over within two minutes of the restart.
A Metcalfe free-kick then led to another warning for the away side as a header from Lions captain Jake Cooper drifted wide.
The Magpies had struggled to impose themselves since half-time, although Harvey Barnes had just their second shot on target after 65 minutes when his tame effort was easily saved by Marschall.
Newcastle boss Matthias Jaissle sent on Anthony Elanga and deadline day signing Matias Fernandez-Pardo in an effort to finally force a breakthrough, but the Millwall centre-back pairing of Cooper and Caleb Taylor were repelling everything coming their way.
Millwall substitute Arconte was the closest to opening the scoring in the 79th minute when he got away from Tino Livramento before his shot came back off the near post.
Having survived that scare, the visitors snatched victory with nine minutes left when Lewis Hall's ball across the six-yard box was flicked in by Willock.
Newcastle defender Lewis Hall speaking to Your Site:
"There was a lot of negativity before everything happened and as soon as the gaffer came in, he's been top, as well as all of the coaching staff.
"The way he's got us playing is amazing and I can speak on behalf of everyone - we're really enjoying playing under him.
"We knew it was going to be tough. Whenever you come away in the cup, anything can happen, so we're really proud of the win and to keep a clean sheet as well."
Newcastle midfielder Joe Willock speaking to Your Site:
"We had a lot of chances in the first half, but we said at half-time that we needed to take our chances. Thankfully, in the second half we did, we got a goal and won the game.
"We won this competition before so it's very important to us. We've got a new coach and a lot of new players and we wanted to start the season well.
"I feel like we started it really well. We need to continue, and this was a big game in keeping that momentum."
Newcastle boss Matthias Jaissle speaking to Your Site:
"We're really happy. Job done. It's never going to be easy here at Millwall in a cup match, so we knew about that challenge and we faced it.
"It was about patience, but also being passionate, being tough, collecting the second balls and being efficient in front of their goal. We used the chance and therefore I'm really proud.
"We matched the physical challenge quite well and made a step in the right direction compared to the first half against Bournemouth. We brought the fundamentals onto the pitch, therefore I'm pleased.
"Joe [Willock] is doing an excellent job. Since we joined, already in pre-season, you saw that there is potential.
"I'm really happy with the start so far. I think we show unity on the pitch and off the pitch already after such a short amount of time. But again, we are realistic. We're happy, we're grateful for that start and now the focus is already towards Leeds.
"I love it [English football]. Honestly, I love it. Coming out for an evening match and knowing that atmosphere is something you dream about. It's a privilege."
