Lautaro Martinez put speculation of a new contract aside to fire Inter in front before Ronaldo emphatically dispatched a 26th-minute penalty.
There was still plenty for Ronaldo to do 10 minutes before the interval when Samir Handanovic charged out to leave his goal unguarded, yet the 35-year-old clipped home with aplomb.
Inter had the better of the second half but were unable to find a response as top scorer Romelu Lukaku served a suspension.
Gianluigi Buffon endured a moment to forget on his 1,100th career appearance as Inter took a ninth-minute lead.
Alexis Sanchez held up play shrewdly for Nicolo Barella, whose measured cross was struck first time by Martinez. Buffon got down in awkward fashion and his touch could not prevent the ball from spinning into his goal.
A VAR review gave Juventus the chance to get back on terms, with referee Gianpaolo Calvarese deciding Ashley Young had impeded Juan Cuadrado enough to award a penalty.
Ronaldo made no mistake, lashing his spot-kick high into the net.
While Handanovic had no chance on that occasion, his inexplicable decision to charge out as Alessandro Bastoni tried to shepherd a hopeful ball away from Ronaldo left the five-time Ballon d'Or winner to slot calmly into an unguarded net.
Young had a drive pushed over by Buffon early in the second half when Juve partially cleared a set-piece, while Handanovic recovered his poise to cleverly keep out a deflected Bernardeschi shot from inside the box.
This was a game where errors continued to enhance the entertainment value. In the 58th minute, a horrible touch from Rodrigo Bentancur coughed up possession to Inter on the edge of the Juve box.
Sanchez looked certain to score, with Buffon stranded, but Merih Demiral made an astonishing goal-line clearance.
Buffon rolled back the years to thwart Matteo Darmian at close quarters following fine work from Sanchez and Martinez, and the latter duo could not fashion an equaliser in Lukaku's absence.