It was a second successive hat-trick for the Belgian, who is playing as an improvised centre-forward following Arkadiusz Milik's long-term knee injury, and took Napoli's goal tally to 13 in their last three league matches.
Napoli's win lifted them to third with 34 points, one behind AS Roma and eight adrift of leaders Juventus.
Lazio later pulled level on points with Napoli by beating Fiorentina 3-1 while Palermo ended a run of nine successive league defeats by scoring twice in the final three minutes to win 4-3 at Genoa.
Mertens became the first Serie A player to score successive hat-tricks for 22 years and the first to score four times in a game since Sassuolo's Domenico Berardi in January 2014.
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It was hard to believe that Napoli had struggled for goals immediately after Milik was injured playing for Poland in early October.
Mertens opened his account in the 13th minute, sending his shot through a crowd of players following a corner, and added the second from a penalty five minutes later.
Torino were all over the place and Mertens completed his hat-trick in the 22nd minute after Jose Callejon's shot was stopped by Joe Hart and the Belgian's first attempt was cleared off the line.
The second half was a different story as Andrea Belotti pulled one back for the visitors just before the hour, Vlad Chiriches scored Napoli's fourth and Luca Rossettini replied for Torino after Pepe Reina failed to hold Adem Ljajic's free kick.
Mertens then produced his best goal of the game with a delightful chip past Hart before Iago Falque pulled another back for ninth-placed Torino with a penalty.
Keita Balde and a Lucas Biglia penalty put Lazio 2-0 ahead at halftime against Fiorentina before the visitors hit back.
Fiorentina's Josip Ilicic missed a penalty before Mauro Zarate pulled one goal back only for Stefano Radu to wrap it up for Lazio in the final minute.
Two goals from Giovanni Simeone, the son of Atletico Madrid coach Diego, helped Genoa to take a 3-1 lead over Palermo before the Sicilians rallied to move off the bottom of the table.
Edoardo Goldaniga headed one goal back in the 69th minute, Andrea Rispoli levelled in the 88th minute and Aleksandar Trajkovski scored a last-minute winner.
Genoa's frustration boiled over when goalkeeper Mattia Perin was sent off for pushing an opponent.
Antonio Candreva's second-half goal gave an unconvincing Inter Milan a 1-0 away win over Sassuolo, who had 10 players on the injured list. Seventh-placed Inter had midfielder Felipe Melo sent off in stoppage time for a second bookable offence.
Bottom club Pescara had Valerio Verre given a straight red card for a dangerous tackle after only 16 minutes as they slumped to a 3-0 home defeat against Bologna.
Bologna, already leading from Adam Masina's header, added further goals with a Blerim Dzemaili shot which took a huge deflection and a Ladislav Krejci penalty.
Cyril Thereau scored twice to give Udinese a 2-0 win over lowly Crotone, their third win a row, while the promoted side's unhappy afternoon was completed when goalkeeper Alex Cordaz was sent off in the 86th minute for handling outside the penalty area.
First-half goals from Riccardo Meggiorini and Sergio Pellissier set up a 2-1 win for Chievo over Sampdoria in Verona.
(Writing by Brian Homewood in Berne; Editing by Clare Fallon)
