Having seen its aspirations of again retaining the Serie A title dealt a blow by a 2-0 loss to rival Inter, Juve could have been considered vulnerable to a Napoli side coming off a 6-0 thumping of Fiorentina.
But Andrea Pirlo's men grew into the game in the second half and Ronaldo rewarded their efforts with what proved to be the decisive goal in the 64th minute.
Pirlo's first title as Juve head coach came with a dramatic conclusion, however, Insigne incredibly chipping wide from the spot after Weston McKinnie had felled Dries Mertens before Napoli were caught cold on an injury-time counter and Alvaro Morata's simple finish sealed the Bianconeri's ninth Supercoppa crown.
An open but low-key first half was light on chances until Wojciech Szczesny had to produce a reaction to keep out Hirving Lozano's diving header from Diego Demme's cross in the 28th minute.
Ronaldo went close with a dipping effort from the left side of the box that narrowly cleared the crossbar 12 minutes later and it was Juve who started the second half brighter.
Davide Ospina had to be alert to prevent Federico Bernardeschi from scoring within a minute of his half-time introduction after good work from McKinnie.
Ronaldo then prodded narrowly wide after getting ahead of a throughball from Arthur, but he made no mistake when Bernardeschi's corner deflected into his path, lashing home from point-blank range.
Insigne had a golden chance to erase the deficit 10 minutes from time after Paolo Valeri pointed to the spot following a VAR review. He sent Szczesny the wrong way but shot woefully wide of the left-hand post.
The excellent Szczesny produced a fine 94th-minute save with his legs to deny Insigne and glory for the Bianconeri was sealed as Cuadrado broke free soon after and laid the pass on for Morata to stroke home.