Napoli are four points ahead of the chasing pack thanks to McTominay's neat finish in the 31st minute, following a trademark dribble into the area from Khvicha Kvaratskhelia.
McTominay's winner was his third Serie A goal since signing for Napoli in the summer and the Scotland midfielder is playing a key role in his new team's bid for a fourth Italian league title.
Napoli would have left Turin with a heftier goal haul had Torino goalkeeper Vanja Milinkovic-Savic not pulled off a series of top class saves.
The Serbia 'keeper kept out a Romelu Lukaku backheel and Kvaratskhelia's looping header before McTominay broke the deadlock, and he got down brilliantly to stop both Mathias Olivera's close-range header in the 65th minute and Giovanni Simeone in stoppage time.
"We attacked them with aggression, much more than usual and it was only Milinkovic-Savic's performance that stopped us from scoring more," Napoli coach Antonio Conte told DAZN.
Atalanta can move back to within a point of Napoli with a win at Roma on Monday night.
Lazio lost ground after a frustrating 3-1 defeat at Parma which was inflicted by an early Dennis Man finish, Anas Haj Mohamed's first Serie A goal and a stoppage-time strike from Enrico Del Prato after Valentin Castellanos had halved the deficit for the away side.
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