Mertens has been pushed into the main striker's role by coach Maurizio Sarri to fill in for missing centre-forward Arkadiusz Milik.
Mertens has been pushed into the main striker's role by coach Maurizio Sarri to fill in for missing centre-forward Arkadiusz Milik, who is scheduled to return from a knee ligament injury in January.
The diminutive Belgium forward has seized the day in astonishing fashion for the southerners, who continue to thrive following the departure of 36-goal hero Gonzalo Higuain to Juventus last summer.
Mertens hit three goals in a 5-0 rout of Cagliari last week to add to a goal he scored against Benfica in a 2-1 Champions League win a fortnight ago.
At the San Paolo on Sunday, the 29-year-old completed his hat-trick, including one from the spot, by the 22nd minute as Napoli laid the foundations for a win that moved them up to third, eight points behind leaders Juventus, at the expense of stuttering AC Milan.
Fresh from a 3-1 defeat to city rivals Juventus last week, Torino coach Sinisa Mihajlovic was so impressed with Napoli he said: "Without Higuain and then without Milik, he's found other solutions.
"I don't know if Napoli are better than the others (teams), but they certainly play better."
For Napoli's film-producer owner Aurelio De Laurentiis, it was the chance to take another shot at Higuain -- whom he accused of "betrayal" last summer despite pocketing 90m euros from Juve for his sale.
"Is it just me or has Mertens scored 10 goals just like Higuain?" De Laurentiis quipped.
Torino left everything on the pitch in a bid to beat Juventus last week, but facing Napoli was like walking into a tidal wave.
Hart was unlucky not to make contact when Mertens struck a first-time shot from just outside the box on 13 minutes following a well-worked corner.
But the Belgian's shot was virtually unstoppable and five minutes later Mertens had his second, sending Hart the wrong way from the spot after being hauled down from behind.
After Hart saved twice, including a spectacular one-handed effort at his near post, the keeper, racing to get back into position, could only look on as Mertens picked up the rebound to fire into the roof of the net.
Although Torino came fighting back thanks to Andrea Belotti's strike on the hour, Napoli restored their three-goal advantage when Vlad Chiriches tapped home from Jose Callejon's delivery on 70 minutes.
Torino hit their second minutes later when Luca Rossettini stepped in to kick the ball away from Pepe Reina's hands as he tried to smother it after spilling an Adem Ljajic free-kick.
But Hart was rooted 10 minutes from the end when an audacious lob by Mertens spun over his head and into the net from a difficult angle.
Mertens has now scored eight goals in his past three league games for Napoli and has 10 league goals this term, two fewer than league-leading trio Edin Dzeko (Roma), Mauro Icardi (Inter) and Belotti.
But Sarri wasn't happy: "The scoreline doesn't reflect the game! That's three or four games where we've missed chances to add goals."
Antonio Candreva struck a second-half winner as Inter Milan edged Sassuolo 1-0 to hand coach Stefano Pioli his first away victory since he replaced Dutchman Frank De Boer, although Inter are still 15 points off top spot.
Juventus, bidding for a record sixth consecutive league title, moved seven points clear of Roma atop Serie A with a 1-0 win over the capital side on Saturday thanks to a sumptuous 14th-minute strike from Higuain.
Keita Balde, Lucas Biglia, from the spot and Stefan Radu struck for Lazio in a 3-1 win over Fiorentina on Sunday that moved the capital side up to fourth at eight points behind Juve.
Palermo, meanwhile, produced a sensational fightback in Genoa to stun the hosts, for whom Giovanni Simeone hit a brace, 4-3 thanks to Aleksandar Trajkovski's 90th minute winner.
The Sicilians' second win of the campaign, and first under new coach Eugenio Corini, moved them up one place from bottom spot.