Longstaff, 19, drilled home a lovely, low finish from outside the box to leave the Red Devils just two points above the bottom three and with two wins this season.
Longstaff, playing in midfield alongside brother Sean, could have scored in the first half but rattled the crossbar with a thunderous 25-yard drive, while Fabian Schar flicked a header narrowly over from a corner.
Ole Gunnar Solskjaer’s side looked bereft of ideas in front of goal: Andreas Pereira’s curling strike was comfortable for Martin Dubravka, while Harry Maguire nodded a free header wide from four yards.
The visitors’ tally of nine points after eight games is their lowest since the 1989-90 season, when they were a point worse off.
Toothless Reds hit new lows
These two Uniteds had some tremendous title tussles during the 1990s, but the situation is much different these days with both sets of fans deeply unhappy with their respective owners.
Though big money was spent in the summer – newcastle signing striker Joelinton for a club record £40m and their opponents bringing in defender Maguire for £80m – both face the prospect of a difficult season.
The Magpies are fighting to remain in the top flight and while the Red Devils are attempting to claim a European place, they find themselves in 12th place in the table.
They last won away from home in the league in February and extended that run to eight games with another lifeless and uninspiring showing.
Their defence has improved from last term, conceding eight times so far which is the third-best record in the division, but it is in attack where the major problems lie.
Solskjaer’s men failed to get a single shot on target against AZ Alkmaar in the europa league on Thursday and again struggled to carve out clear-cut chances in the final third, the best opportunity falling to Maguire, who should have converted his close-range header in the first half.
Playmaker Juan Mata contributed little, while in attack Marcus Rashford received no service in a side that has not scored more than one goal in a game since hitting four against chelsea on the opening weekend.
Magpies much improved
Newcastle boss Steve Bruce had tried and failed to beat his old employers as a premier league manager on 21 previous occasions… but that all changed with this result.
His job was under scrutiny after a 5-0 capitulation at Leicester in their previous game, but Newcastle’s first home win of the campaign moves them up to 16th place.
Bruce, managing in a Premier League game for the 400th time, gave a first top-flight start to Matty Longstaff and his impressive showing was rewarded with a delightful goal.
Longstaff did not look out of place alongside his older brother, making more passes (39) than any team-mate and contributing in defence by winning the ball back four times.
He told BBC Sport: “I think getting to play with Sean was unbelievable. We used to kick each other in the garden growing up so to be on the same team, for our hometown club, and to win against Man Utd, who are a good side, is a bit surreal.
“I found out I was in the team after training yesterday. Your belly starts to turn a bit, you get a bit nervous and then you don’t sleep much. I dreamed about it last night, this is kind how it of went.”
The lively Allan Saint-Maximin dragged a shot wide, Miguel Almiron continues to seek his first goal for the club as a low shot was brilliantly blocked by Maguire, and substitute Andy Carroll steered a diving header over the bar.
Man of the match – Matty Longstaff (Newcastle)
Bruce’s milestone – the stats
- Man Utd have won just nine points from their eight Premier League games this season – their lowest total after the first eight games of a league season since 1989-90 (8 pts).
- Man Utd are winless in their last eight away league games (D3 L5) – their longest such run in the top-flight since September 1989 (11 games).
- Steve Bruce took charge of his 400th Premier League game, the seventh manager to reach this milestone. He’s only the fourth manager to win on his 400th game in the competition (also David Moyes, Alex Ferguson and Arsene Wenger).
- manchester united have won just two of their last 13 Premier League games (D5 L6).
- Ole Gunnar Solskjaer won his first nine away games in all competitions as Man Utd manager, since then he’s won none of his last 11 (D4 L7).
- Matty Longstaff became the youngest player to score on his Premier League debut for newcastle united (19y 199d).
- Matty Longstaff became the second youngest player to score a winning goal against Man U in the Premier League, after Jermain Defoe (19y 62d in December 2001 for West Ham).
- No Man Utd player had more touches in the opposition box (4) or attempted more shots (3) versus Newcastle than defender Harry Maguire.
After the international break, Newcastle travel to Chelsea on 19 October (kick-off 15:00 BST), while it does not get any easier for Manchester United, who host unbeaten Premier League leaders liverpool the following day (16:30).
- 22YedlinBooked at 15mins
- 5SchärBooked at 47mins
- 24AlmirónBooked at 65minsSubstituted forKrafthat 90′minutes
- 43M Longstaff
- 36S Longstaff
- 10Saint-MaximinSubstituted forAtsuat 83′minutes
- 9JoelintonSubstituted forCarrollat 56′minutes
- 1de Gea
- 20DalotSubstituted forRojoat 60′minutesBooked at 78mins
- 18YoungSubstituted forChongat 86′minutes
- 39McTominayBooked at 30mins
- 17FredBooked at 3mins
- 8MataSubstituted forGreenwoodat 66′minutes
- Mike Dean
- Shots on Target
Match ends, Newcastle United 1, Manchester United 0.
Second Half ends, Newcastle United 1, Manchester United 0.
Hand ball by Emil Krafth (Newcastle United).
Substitution, Newcastle United. Emil Krafth replaces Miguel Almirón.
Foul by Tahith Chong (Manchester United).
Andy Carroll (Newcastle United) wins a free kick on the right wing.
Foul by Tahith Chong (Manchester United).
Miguel Almirón (Newcastle United) wins a free kick in the defensive half.
Substitution, Manchester United. Tahith Chong replaces Ashley Young.
Attempt saved. Mason Greenwood (Manchester United) header from the centre of the box is saved in the bottom right corner. Assisted by Ashley Young with a cross.
Substitution, Newcastle United. Christian Atsu replaces Allan Saint-Maximin.
Attempt blocked. Andreas Pereira (Manchester United) right footed shot from outside the box is blocked.
Ashley Young (Manchester United) wins a free kick in the attacking half.
Foul by Sean Longstaff (Newcastle United).
Attempt missed. Fred (Manchester United) left footed shot from outside the box is high and wide to the left. Assisted by Harry Maguire.
Marcos Rojo (Manchester United) is shown the yellow card for a bad foul.
Attempt saved. Sean Longstaff (Newcastle United) right footed shot from outside the box is saved in the bottom left corner. Assisted by Jetro Willems.
Attempt missed. Marcos Rojo (Manchester United) header from the centre of the box is too high. Assisted by Ashley Young with a cross following a set piece situation.
Axel Tuanzebe (Manchester United) wins a free kick on the right wing.
Foul by Miguel Almirón (Newcastle United).
Goal! Newcastle United 1, Manchester United 0. Matthew Longstaff (Newcastle United) right footed shot from outside the box to the bottom left corner. Assisted by Jetro Willems.
Attempt missed. Scott McTominay (Manchester United) header from the centre of the box misses to the right. Assisted by Ashley Young with a cross following a corner.
Corner, Manchester United. Conceded by Andy Carroll.
Corner, Manchester United. Conceded by Fabian Schär.
Substitution, Manchester United. Mason Greenwood replaces Juan Mata.
Miguel Almirón (Newcastle United) is shown the yellow card.
Miguel Almirón (Newcastle United) has gone down, but the referee deems it simulation.
Attempt missed. Andy Carroll (Newcastle United) header from the centre of the box is too high. Assisted by Matthew Longstaff with a cross.
Foul by Marcos Rojo (Manchester United).
Andy Carroll (Newcastle United) wins a free kick in the defensive half.
Corner, Manchester United. Conceded by DeAndre Yedlin.
Attempt blocked. Scott McTominay (Manchester United) right footed shot from outside the box is blocked. Assisted by Marcus Rashford.
Substitution, Manchester United. Marcos Rojo replaces Diogo Dalot.
Hand ball by Scott McTominay (Manchester United).
Attempt saved. Daniel James (Manchester United) right footed shot from the right side of the box is saved in the bottom right corner. Assisted by Marcus Rashford.
Foul by Scott McTominay (Manchester United).
Sean Longstaff (Newcastle United) wins a free kick in the defensive half.
Substitution, Newcastle United. Andy Carroll replaces Joelinton.
Attempt blocked. Harry Maguire (Manchester United) right footed shot from the centre of the box is blocked. Assisted by Scott McTominay.
Attempt missed. Harry Maguire (Manchester United) header from the centre of the box misses to the right. Assisted by Ashley Young with a cross following a corner.