The televised clash was something of a slow burner to begin with, but sprung to life in a remarkable 15-minute period leading up the break, Karl Darlow keeping out two Forest spot-kicks either side of Matt Ritchie’s opener.
Both Jonjo Shelvey and Paul Dummett were dismissed for Newcastle, but neither Nicklas Bendtner nor Henri Lansbury could get the better of Darlow from 12 yards.
Bendtner did finally find a way past the stopper early into the second half, before a courageous rearguard effort from the visitors just fell short, as Cash bundled home four minutes from time.
A Forest side buoyed by back-to-back victories were first to go close. Eric Lichaj swung in a fine cross from the left, and – following a well-timed run – young midfielder Cash saw a close-range effort pushed wide by Darlow.
Just seconds later, United threatened for the first time themselves. Shelvey picked out Ritchie after DeAndre Yedlin’s fizzing cross was only half-cleared, and the Scot sent the ball narrowly wide from 20 yards.
After a cagey period, Yoan Gouffran curled off-target from similar range after gathering Gayle’s lay-off.
Gayle himself then saw a deflected header drop wide after meeting Yedlin’s cross from the right, Newcastle beginning to turn the screw as the first half progressed.
But, just after the half-hour mark, Rafa Benitez’s side were dealt a hammer blow, as Shelvey – their brightest spark in the opening exchanges – was dismissed following an off-the-ball incident with Lansbury.
Forest were awarded a spot-kick as a result, but former Arsenal frontman Bendtner was unable to beat Darlow – himself a regular at the City Ground prior to joining United.
Ciaran Clark then saw a headed effort ruled out, before Gouffran’s stinging attempt was saved by Vladimir Stojkovic at his near post.
Despite playing a man light, United were actually looking the brighter side, and – with 45 minutes on the clock – they broke the deadlock, as Ritchie powered home after pouncing on Isaac Hayden’s deflected cutback.
More drama was to come. Seconds into added time, Dummett was given his marching orders after bringing down Lansbury inside the United area, but Darlow came to the rescue again – diving low to his right to parry the Forest skipper’s penalty.
52 minutes in, however, the home side finally found a way through.
Substitute Britt Assombalonga kept the pressure on following a low cross, and Bendtner eventually bundled home with Darlow powerless.
The Dane then skewed wide from Thomas Lam’s driven cross, before Mustapha Carayol’s powerful strike was well held by the Magpies’ stopper.
With just over a quarter-of-an-hour remaining, Bendtner’s goalbound header was flicked clear by Yedlin. Assombalonga then took aim, but Jamaal Lascelles stood firm to block.
Unsurprisingly, Newcastle were reduced to slim pickings at the other end, though Hayden did bend a right-footed attempt narrowly wide of the far post from just inside the area.
Time looked to be getting away from the home side, but, with just minutes of the original 90 remaining, United’s defence was breached for a second time.
Lansbury’s corner from the left took a touch en route to Cash at the back post, who forced the ball home to break the Magpies’ hearts.
A valiant effort nonetheless from Benitez’s charges, who return to Sky Bet Championship action against Birmingham City at St. James’ Park next Saturday.
Nottingham Forest: Vladimir Stojkovic; Eric Lichaj, Michael Mancienne (Britt Assombalonga 46), Matt Mills, Damien Perquis; Matty Cash, Thomas Lam (Mustapha Carayol 65); Henri Lansbury, Ben Osborn, Patjim Kasami; Nicklas Bendtner (Apostolos Vellios 79).
Substitutes: Stephen Henderson, David Vaughan, Jack Hobbs, Jorge Grant.
Newcastle United: Karl Darlow; DeAndre Yedlin, Ciaran Clark, Jamaal Lascelles, Paul Dummett; Isaac Hayden, Jonjo Shelvey; Matt Ritchie, Ayoze Perez (Vurnon Anita 46), Yoan Gouffran (Mohamed Diame 77); Dwight Gayle (Aleksandar Mitrovic 87).
Substitutes: Matz Sels, Chancel Mbemba, Cheick Tiote, Christian Atsu.
Referee: Stephen Martin
Bookings: Nottingham Forest – Michael Mancienne (44), Henri Lansbury (72); Newcastle United – Karl Darlow (45+3), Yoan Gouffran (53).
Red Cards: Nottingham Forest – ; Newcastle United – Jonjo Shelvey (33), Paul Dummett (45+1).
