There were just 23 seconds on the clock when City grabbed the lead.
Newcastle had actually kicked the game off before a casual punt forward from David Silva picked out Raheem Sterling before the United defence appeared to freeze.
Sterling got free down the right before curling the all to the back post, where a sprinting Silva continued his involvement in the move to beat Martin Dubravka to the ball before Sergio Aguero applied the finish.
It was a moment that left Benitez raising his arms in utter disbelief as City were handed a gift-wrapped goal so early in the game.
The early goal left United shell-shocked as City dominated possession with 78% of the ball in the first 10 minutes.
But United finally had a glimpse of goal on 13 minutes as Ayoze Perez got free down the right before rolling a shot wide of Ederson’s right-hand post.
The moneybags Manchester side had the ball in the net again four minutes later as Aguero fired home following a crafty free-kick from Kevin De Bruyne – or so he thought.
Indeed, referee Paul Tierney was quick to chalk the goal off and book the Belgian after the set-play was taken too quickly, much to the relief of the Geordie fans.
Mid-way through the first half, United had another sniff of goal as Salomon Rondon knocked the ball down for Christian Atsu but his effort was deflected over.
The visitors seemed to lack intensity as the game drifted to half-time but Sterling still fired in another shot at goal on 36 minutes which was put out of play by Jamaal Lascelles.
United escaped further first half punishment as Danilo skipped down the left before dinking the ball into the box for Silva, but his header was cleared on the line thanks to a combination of Schar and Lejeune.
For all Newcastle’s defensive work, they ended the half with four attempts at goal but none on target.
Nevertheless, they were still in the game going into the second half on a freezing night at St James’ Park.
City had an opening on 49 minutes as Leroy Sane rolled the ball down the left-hand side but nobody connected and United were able to breathe again.
On 58 minutes a slick City move resulted in Silva finding space in the box after De Bruyne’s pass but Dubravka beat the ball to safety.
Just after the hour mark Guardiola’s side were lucky to get away with a red card after De Bruyne steamed into a tackle on Ritchie but after thinking long and hard Tierney controversially gave the title challengers the benefit of the doubt.
However, the roof was raised at the Gallowgate End as Newcastle got forward down the left-hand side with 67 minutes gone.
Ritchie clipped in a cross which was half-cleared and Isaac Hayden looped a header back into the box before Rondon got enough on the ball to send it flying past Ederson and level the game up.
Guardiola responded by throwing on a £27million substitute for Sane with 18 minutes to go.
It set up a tense finale in which Schar was booked for a badly timed challenge on Silva as Newcastle looked to both fair means and foul to try to grab a big result.
Sean Longstaff before he was upended in the box by Fernandinho to win a penalty with 12 minutes left.
Tierney pointed straight to the spot but drama followed as Ederson milked an injury to delay the penalty.
Yet Ritchie showed nerves of steel before slamming the ball hard and low to beat the Brazilian and make it 2-1 to send the Geordie crowd into absolute raptures.
Newcastle had to hold on for a frantic 10 minutes but held their own against Guardiola’s side to spark a Tuesday night party in this football mad city.