The game started without VAR due to techincal issues - however, by the 12th minute these had been resolved.
Roma, who are in eighth place, led when Stephen el Shaarawy converted Cengiz Under's cross.
But Mertens equalised when he turned home Jose Callejon's scuffed shot from Lorenzo Insigne's cross.
Earlier in the day, Suso scored a winner as AC Milan beat Sampdoria 3-2 in an entertaining game.
This result leaves Napoli in second place, six points behind runaway leaders Juventus.
Meanwhile, Roma are languishing in 8th place, having won four games this season.