The Portuguese tapped home from close range five minutes into the second half before beating Andrea Consigli with a left-footed finish 15 minutes later.
He should have had a hat-trick but failed to convert two golden opportunities in the latter stages of the match before Khouma Babacar pulled one back in injury time with a header.
Substitute Douglas Costa was sent off moments later for elbowing, headbutting and spitting at Federico Di Francesco, but it was too late for the visitors to capitalise on their man advantage.
Ronaldo started up front alongside Paulo Dybala and Mario Mandzukic. The lineup was likely a statement of intent from manager Massimiliano Allegri, though things did not go to plan for Ronaldo or Juve in a frustrating first half.
The 33-year-old's first sight of goal came after around six minutes, but his shot was deflected out for a corner. Soon after, he was on the receiving end of a cross but could only head it wide, prompting him to take his frustration out on the turf.
Juve had little else in the way of clear-cut opportunities during a physical first 45 minutes, which saw four yellow cards dished out.
At the other end, Alfred Duncan put the ball past Wojciech Szczesny with a fine finish, but the goal was disallowed for a foul on Joao Cancelo by Filip Djuricic in the buildup.
Ronaldo's only other attempt of note in the first half was a free-kick from 35 yards, but his wayward effort failed to trouble the visitors' goal.
As ESPN's Matteo Bonetti noted, though, the forward's focus is not on Serie A this season:
Nevertheless, he was clearly delighted to get off the mark shortly after the break when he took advantage of Gian Marco Ferrari's failure to clear his lines:
Eleven Sports @ElevenSports_UK
He's off the mark! ＠Cristiano won't care that it's probably the easiest goal of his career. He is up and running and his three-and-a-half game drought (!) is over. It's ＠juventusfcen 1 - ＠SassuoloUS 0. Up and running! Subscribe - https://t.co/YbI6T6O8MU #CR7 #Ronaldo #Juve https://t.co/IXWfTcdRI2
Quarter of an hour later, he had his and Juve's second of the match.
A quick counter-attack saw Emre Can feed Ronaldo on the left, and he made no mistake:
The former Real Madrid man had two gilt-edged chances to bag his hat-trick as the game opened up.
He could only volley Costa's cross wide from six yards before firing a rebound wide from the same distance after Consigli parried a shot from Cancelo straight into his path.
Sassuolo were able to earn a late consolation through Babacar, but Ronaldo will be grateful they didn't manage to punish his uncharacteristic wastefulness late on.
As for Costa, he could be set for a lengthy spell on the sidelines for the unsavoury scenes he caused at the end:
Juve play Napoli on September 29, so a three-match ban or longer would see him miss the early title-race clash.
Ronaldo will be in action for Juventus again on Wednesday, when they travel to Valencia in the UEFA Champions League. The Bianconeri return to Serie A duties against Frosinone on September 23. Sassuolo's next outing sees them take on Empoli on Friday.