The Carabao Cup holders trailed inside seven minutes at the Etihad Stadium as Marmoush exchanged a quick one-two with Phil Foden but despite Dan Burn's sliding tackle, the ball bounced kindly off the Egyptian forward before looping over Magpies goalkeeper Aaron Ramsdale.
Joe Willock, on target in last week's 1-1 draw against Paris Saint-Germain in the UEFA Champions, missed an opportunity to draw United level in swift fashion. After Anthony Gordon made a positive run in behind the City defence before squaring the ball into Willock's path, the midfielder attempted to round James Trafford but the City goalkeeper quickly dived at his feet to claim the ball.
Anthony Gordon went close to netting his 11th goal of the season across all competitions. After Ramsdale's goal kick was headed towards goal by Burn and Willock, the latter's flick on found Gordon but the left winger's low first time effort was blocked out for a corner by a sprawling Trafford.
Newcastle conceded a second goal in the 29th minute as Marmoush doubled his and City's lead. After Tijani Reijnders completed a lengthy run before finding Antoine Semenyo's run, the Ghanian's low cross was hooked into the air by Kieran Trippier before Marmoush rose high inside the six-yard area to nod the ball into an empty net.
United suffered a further blow as Reijnders scored four minutes later, completing a dangerous attack after Semenyo initially checked inside where the former was on hand to place the ball beyond Ramsdale.
Magpies head coach Eddie Howe made three changes to the starting XI as Aaron Ramsdale, Sven Botman and Nick Woltemade replaced Nick Pope, Harvey Barnes and Anthony Elanga respectively.
Backed by over 5,300 away fans tonight, the Magpies' last away win against Manchester City was recorded in October 2014 as goals from Rolando Aarons and Moussa Sissoko sealed a memorable 2-0 victory in the League Cup fourth round.
**Newcastle United:** Aaron Ramsdale, Kieran Trippier (c), Lewis Hall, Sven Botman, Sandro Tonali, Anthony Gordon, Malick Thiaw, Nick Woltemade, Joe Willock, Dan Burn, Jacob Ramsey.
**Substitutes:** Nick Pope, Yoane Wissa, Harvey Barnes, Will Osula, Anthony Elanga, Jacob Murphy, Alex Murphy, Leo Shahar, Sean Neave.