The 18-year-old Mayulu was given a rare game on Saturday as coach Luis Enrique rested players ahead of Tuesday's Champions League game against PSV Eindhoven.
The left midfielder put the defending champions ahead in the 18th minute when he rifled a powerful shot after swapping passes with Barcola down the left flank.
Enrique was without forward Randal Kolo Muani after he hurt an ankle in training and, with Eindhoven in mind, benched captain Marquinhos and speedy winger Ousmane Dembélé at Parc des Princes.
Former Real Madrid midfielder Marco Asensio made it 2-0 from close range in the 47th and, after striker Sékou Mara pulled one back, Barcola slotted in his league-leading seventh goal in the 65th.
South Korea forward Lee Kang-in added another PSG goal, before sloppy defending allowed the visitors to pull one back in stoppage time through 18-year-old Senegalese midfielder Pape Daouda Diong.
PSG and Monaco are both unbeaten with 20 points from eight rounds. Monaco drew with Lille 0-0 on Friday.
In other Saturday action, Portuguese forward Jota equalised in the 86th from Arnaud Kalimuendo's cross to earn Rennes a 1-1 draw at Brest in the Brittany derby.
Romain Del Castillo's penalty put Brest ahead early in the second half following a handball.
Lens snapped a run of five straight league draws with a 2-0 win at Saint-Etienne to move up to fifth place.
Poland winger Przemysław Frankowski netted with a 20-metre strike in the first half and striker Rémy Labeau Lascary grabbed his first senior goal late on.
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