The Ciucciarelli controlled the first half, and although Roma tried to stage a comeback in the second, the experienced Belgian made the difference for the visitors.
The Neapolitans took the lead in the 27th minute when Piotr Zielinski turned and was fouled by Roger Ibanez, who received a yellow card. Mertens took the resulting free-kick and the ball bent around the wall, then into the net at the far post.
Lorenzo Insigne switched the ball to the right-wing, and Matteo Politano headed it across the goal towards the path of Mertens, who headed it at the goal. Roma goalkeeper Pau Lopez dived in vain and failed to keep the ball out despite getting a touch.
The Giallorossi went straight on the attack at the start of the second half, with Lorenzo Pellegrini heading directly at Ospina. Pedro had a chance to pull a goal back in the 57th minute but he blasted the ball high.
Pellegrini hit the post with another shot while Zielinski shot straight at Pau Lopez. Roma substitute Carles Perez also tried his luck but he failed to hit the target.
Lupi wing-back Rick Karsdorp attempted a cross than inadvertently turned into a shot and Ospina had to push the ball over the bar.
Napoli striker Victor Osimhen came on in the second half and he was given a yellow card after an altercation with Roma defender Gianluca Mancini, with the ex-Atalanta man fortunate to avoid being shown a red card.
The victory means that Napoli are now outright fifth in the Serie A table with 53 points whereas Roma are three points behind in sixth place.