Ronaldo stopped a bright Juve opening in its tracks as he swept in Dani Carvajal's pass after 20 minutes, before Mario Mandzukic levelled with one of the great Champions League final goals, firing a sublime overhead kick beyond Real goalkeeper Keylor Navas.
Real were unstoppable after the break, restoring their lead through Casemiro's deflected shot before Ronaldo scored at the near post from a cross by the outstanding Luka Modric.
Juventus' misery increased when substitute Juan Cuadrado was sent off for a second caution after a clash with Sergio Ramos, before substitute Marco Asensio emphasised Real's superiority with a precise finish in the last minute.
Real coach Zinedine Zidane has now won back-to-back Champions Leagues - as well as this season's La Liga title - since he took charge in January 2016.
Ronaldo rules again
Ronaldo cut a frustrated figure early in the match, gesturing to German referee Felix Brych about what he perceived as some rough treatment.
But he ended the night as Real's hero, winning his third Champions League winners' medal for the Spanish club and fourth in total, as the 32-year-old continues to write new chapters in his glittering career.
On display in Cardiff were all the facets that make him an all-time great - pace, desire, anticipation, technique - as he helped Real dismantle a Juve defence that conceded just three goals in 1,080 minutes en route to the final.
Ronaldo's first was an instant sweeping finish beyond the outstretched right arm of Gianluigi Buffon while the second showed the touch of a poacher, moving like lightning beyond static Juve defenders to guide in an emphatic near-post finish.
Cristiano Ronaldo was lifted into the air by his team-mates after Real Madrid's latest European triumph
The Portugal captain now has a remarkable record of 105 goals in 140 Champions League games - 12 of those coming in just 13 games this season.
Big players define big matches and Ronaldo defines more than most.
He did so again here in Cardiff.
Real show their class
Real had their qualities questioned in their most recent Champions League finals, starting with their victory over Atletico Madrid in Lisbon in 2014 - when they needed Ramos' injury-time equaliser to force extra time before going on to win - and again last season when they beat the same opponents on penalties.
No such questions can be levelled at Real after this latest triumph.
This was the performance of a world-class team, ripping apart a defence that has a reputation as one of football's meanest and did not concede a goal in either leg of their quarter-final win against Barcelona, keeping out Lionel Messi, Neymar and Luis Suarez.
Gareth Bale, who returned from a calf injury as a 77th-minute substitute, lifts the Champions League trophy in his home city of Cardiff
Real survived a whirlwind Juventus opening before taking control after the break, with Modric and Toni Kroos manipulating possession in midfield and Ronaldo providing the cutting edge.
Juventus had performed with such distinction in this Champions League that many experts understandably had them as favourites - instead they ended heavily beaten by a ruthless and wonderfully gifted side.
Juventus come up short once more
Juventus looked the complete package en route to this final. Not only did Massimiliano Allegri's side have that formidable defence but also a potent attack led by Gonzalo Higuain.
They started as if determined to banish the demons that have visited them in past Champions League finals. They made a razor-sharp start - Miralem Pjanic's early 25-yard shot beaten away by Navas.
Juve were right in the game after Mandzukic's triumph of technique and athleticism pulled them level - but they faded badly in the second half and ended overwhelmed.
Mario Mandzukic scored a sublime overhead kick goal to equalise for Juventus
And most sympathy will go to Buffon, a three-time loser at 39 years and 126 days and whose expression at the conclusion suggested he knows another chance could be beyond him.
The cruelty of the game was illustrated by the agony on his face as he reached in vain for Casemiro's long-range shot, which was deflected tantalisingly out of his reach off Sami Khedira's heel.
Juve, though, can have no complaints and have now lost five Champions League finals since they last won the competition in 1996.
More about: #RealMadrid #Juventus
