Neumann, who was released by Adelaide after he was the club’s leading scorer at the end of last season, jagged the winning goal to hand United back-to-back A-League defeats after a 1-0 loss to Brisbane Roar at home last week.
Neumann made the most of David Carney’s good work when he ghosted a ball inside the box to smack an angled shot into the back of the net nine minutes before time.
Neumann kept his celebrations in check.
Neumann’s goal gave the Jets their first win of the season just as Adelaide appeared to be on its way to another road victory after Craig Goodwin equalised against his old club in the 69th minute.
“I am really happy with all the effort from all of my teammates,’’ Neumann said.
“I am really happy for the first win of the season and I hope they we have more wins like this.
“Of course I have part of my heart in Adelaide and I am never going to forget the people and my ex-teammates and I am really happy right now.”
Neumann’s goal also gave former Adelaide United coach Phil Stubbins his first victory as a head A-League coach after the Jets last recorded a win — 2-0 — ironically over the Reds in April at the same venue.
It was Scott Neville who first shocked Adelaide five minutes after the break to give the Jets a 1-0 lead.
The Reds appeared sluggish in clearing their lines from a corner before former Red Zenon Caravella whipped in a diagonal ball inside the box which was flicked on by stopper Adrian Madaschi as Adelaide was moving out of defence.
Neville was first to react and poked the ball past Eugene Galekovic.
Neville’s goal arrived after the Jets threatened to open the score just before half-time and it was the cat-like reflexes of Galekovic which saved Adelaide from conceding.
The Reds captain made an outstanding goal-line save from an Andrew Hoole point-blank header after a tantalising Carney cross.
The rebound fell invitingly for on-loan Barcelona SC striker Edson Montano but the Ecuadorean smacked his volley into the back of Hoole’s legs.
But Adelaide will be rather disappointed that it couldn’t take at least a point off the Jets after Goodwin scored when Awer Mabil caused the damage on the right flank.
Jimmy Jeggo ran onto a Mabil flick to release a ball across the box to give Goodwin a tap in.
Adelaide then camped in the Jets half but failed to find a winner in an attempt to sour the Jets “retro night” and its 10-year anniversary.
Adelaide exposed the nervous home defence through the brilliance of Fabio Ferreira, the class of Marcelo Carrusca and the speed of FFA Cup hero Sergio Cirio.
The Jets were caught out many times but United failed to punish the side which until kick-off hadn’t claimed an A-League win this season.
Pablo Sanchez — in for the injured Bruce Djite — was presented with three gilt-edged opportunities in the first half.
His first chance arrived when Ferreira made space on the flank before whipping in a cross which Sanchez got to but he smacked the ball into stopper Madaschi in the third minute.
Two minutes later Ferreira cut inside former Croydon Kings defender Allan Welsh before his 12m left foot drive whistled past the upright.
Carrusca then floated in a perfect 40m ball for Sanchez which isolated Jets defenders but the Spaniard couldn’t bring the ball down quickly enough before gloveman Ben Kennedy snatched the leather.
But after Sanchez couldn’t get on the end of a Goodwin cross after Cirio sped away from the Jets defenders Newcastle started to gain confidence.
Montano tested Galekovic with a 15m drive after James Virgili penetrated Adelaide’s last lines in the 33rd minute as United started to look rather fatigued perhaps due to FFA Cup commitments just 72 hours earlier.
At the half-time break Ferreira surprisingly ended his shift for Mabil but it was the Jets taking the game to the visitors a minute after the break.
Montano again forced Galekovic into a wonderful save when his goal-bound header from a Virgili cross was turned around the post for a corner.
Adelaide reverted to a three-man defence when Mabil entered the fray but the Jets continually frustrated the Reds’ free-flowing game.
Mabil did get a cross in which troubled the Jets defence in the 62nd minute but Ben Kantarovski cleared the danger with Sanchez lurking.
NEWCASTLE JETS 2 (Scott Neville 49m, Jeronimo Neumann 81m) bt ADELAIDE UNITED 1 (Craig Goodwin 69m) at Hunter Stadium. Crowd: 10,407. Referee: Strebre Delovski