PSG's title chances take another blow
Mohamed Sissoko got PSG back on level terms with a 50th-minute equaliser after Bakaye Traore had put the hosts ahead in the 18th minute, but Mollo coolly finished off a counter-attack two minutes from time to stun the visitors.
Carlo Ancelotti's team were looking to put recent disappointments behind them after draws against Caen and Bordeaux allowed Montpellier to depose them at the top of the table on goal difference.
Nancy started well, though, with PSG goalkeeper Salvatore Sirigu making the first save of the match from Daniel Niculae's speculative effort in the 13th minute.
And Sirigu was beaten five minutes later as Traore despatched a right-footed shot into the net after good work by Mollo down the left.
The home team went within a whisker of doubling their advantage in the 21st minute, Andre Luiz's fierce 20-yard blast smashing against the crossbar.
PSG had their first sniff moments later and nearly equalised, Jeremy Menez heading inches wide after Javier Pastore had latched onto Sissoko's pass and set up the chance.
Sirigu had to be alert three minutes before the break to keep his side only one goal down, catching Niculae's header from Mollo's free-kick.
Nancy, looking to add PSG to the scalps of Lyon and Montpellier, had another chance to go two up in the opening moments of the second half, but Djamel Bakar's curling left-footed effort was saved by Sirigu.
And within five minutes of the restart PSG were level as Sissoko headed powerfully home from Sylvain Armand's left-wing cross.
Armand almost put his side ahead just before the hour with a stunning left-foot volley from the edge of the area, but Nancy goalkeeper Guy N'dy Assembe got a firm hand to it to ensure the only damage was a corner.
N'dy Assembe was again in the right place two minutes later to keep out Guillaume Hoarau's header from Christophe Jallet's cross, and then denied Menez soon after as PSG pushed forward in search of the winner to regain top spot.
However, they could find no way past and paid the price for their ambition as Nancy nicked the winner themselves on an 88th-minute breakaway.
Benjamin Moukandjo supplied the pass for Mollo on the edge of the box and he calmly placed his left-footed shot past Sirigu.
Information from The Associated Press was used in this report.