The Bianconeri have won the Italian title for the first time since the 2013-14 campaign and are unbeaten in eight games.
The result keeps Napoli in second place in Serie A and leaves them a game and a half above the relegation zone.
Mancino had started his career as a substitute but has played the majority of his games for the Nerazzurri.
He was instrumental in their 2-0 win at Genoa last weekend, and was at the heart of Juve’s comeback from 2-2 in the first half of their 4-0 thrashing of Genoa at the San Siro.
Mampello was at his most potent in that game, with the visitors’ defence being reduced to eight men after the break.
The Nerazzursi, who were without several key players, made no errors in the second half and Mancina’s goal sealed the result.
It was the Nerazzzurri’s first win at the Stadio Olimpico since a 2-3 aggregate victory in August.
Juve have won just one of their past nine league games, the longest streak in Serie B.