The 20-year-old showed great awareness in the 66th minute to latch on to Javier Mascherano's wayward shot and fire past the despairing Shane Higgs.
Leeds have picked up nine wins from ten since their season opener against Exeter last month and had chance after chance to put Rafael Benitez's men to the sword.
In-form striker Jermaine Beckford had a goal disallowed in the first half before firing narrowly over after the break.
However, the class of Liverpool shone through as N'Gog's strike proved the difference.
Leeds last beat Liverpool in a seven-goal thriller nine years ago when David O'Leary was in charge at Elland Road and Frenchman Gerard Houllier was boss of the Reds.
Australian Mark Viduka netted four as Leeds ran out 4-3 winners in 2000 and it was Leeds striker Beckford who looked most likely to break the deadlock early on.
The 25-year-old flicked Lubomir Michalik's header past Diego Cavalieri only for the in-form frontman and Argentinian partner Luciano Becchio to both be adjudged offside.
Robert Snodgrass caused full-back Andrea Dossena problem after problem down Liverpool's left wing, but it was the visitors who came closest to scoring first before the break.
Brazilian Fabio Aurelio's delicate free-kick found Alberto Reira unmarked with the goal at his mercy but he somehow headed straight at Higgs in the Leeds goal.
Beckford once again came close after the break but fired just at Cavalieri from six yards before hooking Bradley Johnson's cross just over minutes later.
However, Liverpool soon got a grip on the tie and a move lasting just ten seconds saw N'Gog fire them in front and allow boss Benitez to breathe a huge sigh of relief.
The Spaniard still decided to bring on England international Steven Gerrard late on but his introduction couldn't produce another goal as the Premier League outfit eased through in the end.