Marcelo Bielsa's team were on course to regain top spot from Lyon after coming from behind to lead 2-1 going into the final moments of Friday's game.
However, former Liverpool and Bolton Wanderers striker N'Gog came off the bench to equalise with a well-taken header in the 90th minute to ensure Marseille remain second.
Leaders Lyon can now move three points clear of Bielsa's side with victory at Lorient on Sunday, while champions Paris Saint-Germain will go second if they beat Caen on Saturday.
Reims forward Nicolas de Preville opened the scoring after just six minutes, thanks in the main to some suspect goalkeeping from Steve Mandanda.
Mandanda could only parry Prince Oniangue's speculative shot straight into De Preville's path, and the 24-year-old gratefully slotted home just his second Ligue 1 goal of the season.
Unsurprisingly, Marseille then laid siege to the Reims goal in search of an equaliser, with Dimitri Payet at the heart of everything positive the hosts did.
First, the former Lille man had an excellent 25-yard free-kick tipped wide by Kossi Agassa, before he then supplied an inch-perfect cross for Andre Ayew, who could only head wide from close range.
Marseille picked up where they left off after the break, but De Preville missed a golden chance to double their arrears when he curled wide after an incisive Reims breakaway.
And De Preville was made to pay for that miss when, somewhat inevitably, Payet equalised in the 58th minute - albeit with the help of a huge deflection, which resulted in his effort from 18 yards completely wrong-footing Agassa.
Marseille continued to press and got their just rewards when Ayew slammed home from a yard when Reims failed to deal with a corner 11 minutes after Payet's leveller - sparking delirious scenes among the Velodrome faithful.
Gaetan Charbonnier gave Bielsa an almighty scare when he rattled the crossbar from close range with 20 minutes remaining, but it looked like Marseille had done enough to take away a deserved three points.
However, N'Gog had other ideas, as he steered Charbonnier's superb cross beyond Mandanda to seal a share of the spoils for Reims.
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