The Premier League giants were outplayed by the League One leaders for much of the third-round tie but Ngog netted against the run of play after 65 minutes.
Leeds spurned a number of chances throughout the game as Elland Road enjoyed a big-game atmosphere once again but their lack of composure in front of goal cost them.
Benni McCarthy's fortuitous free-kick ensured a successful return to Nottingham for Blackburn boss Sam Allardyce as Rovers reached the fourth round with a slender 1-0 victory at the City Ground.
Allardyce, who managed Forest's city rivals Notts County between 1997 and 1999, guiding the Magpies to the old Third Division title by a staggering 19 points in his second season, saw his side score against the run of play in the 37th minute.
Martin Olsson had been brought down on the left side of the area. McCarthy's curling right-foot cross appeared to evade everyone in the box and nestle in the far corner beyond Paul Smith's desperate dive.
It was harsh on the Coca-Cola Championship club who created the better chances throughout and missed a second-half penalty when Jason Brown superbly saved Dexter Blackstock's kick.
Peterborough sent Newcastle crashing out 2-0 thanks to first-half goals from Craig Mackail-Smith and Tom Williams.
Scunthorpe needed extra-time to beat League Two Port Vale 2-0 and progress to the fourth round for the first time in their history at Glanford Park.
Having wasted a plethora of opportunities throughout the 90 minutes it took them just two minutes of the extra period to finally open the scoring through Paul Hayes. And they put the game to bed just three minutes later when Grant McCann finished off a neat move to dink the ball home.