The £100m off-season signing from Real Madrid netted a towering header just after the hour mark in Saudi Arabia, while Milan were reduced to ten-men late on in the fixture after Ivory Coast midfielder Frank Kessie received his maraching orders.
The Italian version of the Super Cup marks the first medal for Ronaldo as a Juventus player since his club-record switch and his 29th overall for club and country.
"It was a very difficult match, it's very warm and hard to play in these conditions," he told Rai Sport. "We played well, created a lot of chances and obviously I am happy to have scored the winning goal.
"It was my intention to start 2019 with a trophy, I have my first title with Juve and I am very happy. But it is only the start, we will take it one step at a time. We've got this trophy, now we must keep working hard to achieve the next one."
With the Bianconeri nine points clear of second-place Napoli within the Italian top-flight, Massimo Allegri's side are overwhelming faviorites to secure their eighth consecutive league title - which would added to the growing list of honours in Ronaldo's already decorated career.
But the Portugal captain, whom teammate Girgio Chillieni recently stated is 'the final piece of the puzzle' to compete for every honour on offer, is not ready to allow complacency to creep in.
"Serie A is always Juve's main objective," Ronaldo added. "We are top of the table, but it's a very long tournament and it's going to be tough, so we need to keep working."