Gli Azzurri had enjoyed an encouraging start to the contest but found themselves behind in the ninth minute.
Stanislav Lobotka’s risky pass allowed Piotr Zielinski to dispossess McTominay, starting a move that culminated in Marcus Thuram laying the ball off for Federico Dimarco to smash a pinpoint strike in off the inside of the post.
Inter kept coming with some eye-catching link-up play carving out a chance for Lautaro Martinez, although his scuffed attempt was routinely gathered by Vanja Milinkovic-Savic shortly before Hakan Calhanoglu fired an ambitious shot off-target.
Having come through that pressure without any further damage being done, Napoli struck back with a nicely-worked goal of their own in the 26th minute as McTominay reached Matteo Politano’s low cross ahead of Manuel Akanji to apply a clinical first-time finish.
McTominay then sprinted to dispossess Zielinski with a one-man press but saw his shot denied by Yann Sommer. Despite initially looking knocked by the equaliser, Inter came forward again before the break but Zieliński’s sweetly-struck shot hit Martínez, before Thuram and Yann Bissek had opportunities before half-time.
Napoli had two major chances within five minutes of the restart, but Rasmus Hojlund’s shot grazed the post on its way behind and Giovanni Di Lorenzo made a complete mess of his header.
Nicolo Barella smashed a speculative effort harmlessly off-target before Højlund and Calhanoglu had an effort from range blocked at either end.
Inter then had their chance to go ahead in the 73rd minute after Amir Rrahmani trod on Henrikh Mkhitaryan's foot, and Antonio Conte’s furious response to the penalty being given earned him a red card before Calhanoglu compounded any frustrations by firing his spot kick in off the post.
Their lead lasted just eight minutes, though, as Noa Lang guided Politano’s cross back into a dangerous area for McTominay to score his second equaliser of the day.
Mkhitaryan then hit the post with a deflected shot in stoppage time as Napoli earned their second consecutive draw, which at least sees them reclaim third place. While the goal ended I Nerazzurri’s six-game winning run in Serie A, they are still top of the table by three points.