Ronaldo, who was sent off in Wednesday's Champions League win at Valencia, looked set to endure another frustrating evening until he scuffed home a left-footed effort with nine minutes remaining.
The Portuguese forward squandered a series of chances at Stadio Benito Stirpe before putting the Italian champions on course for a fifth league win from five this term with his third goal for his new club.
Substitute Bernardeschi added a second in injury time after Juve broke away on the counter-attack.
Lowly Frosinone, who have registered just one point following promotion, produced a dogged display but rarely ventured forward as their search for a first goal of the campaign continues.
Juve were without winger Douglas Costa as he began a four-match ban after his red card for spitting in last weekend's 2-1 win over Sassuolo.
Ronaldo scored his first Juve goals in that game following his big-money summer move from Real Madrid and he retained his starting place despite his midweek red card.
The Portuguese went close to a seventh-minute opener when his close-range effort from Mario Mandzukic's knockdown brushed the fingertips of goalkeeper Marco Sportiello and was cleared off the line by defender Marco Capuano.
Second-bottom Frosinone came into this fixture on the back of a morale-sapping 5-0 thrashing at home to Sampdoria.
They again had to be alert to the talents of Ronaldo when he fired a low effort towards the bottom left corner which was repelled by Sportiello.
Spurred on by a vocal home support, the hosts reached half-time without too many scares although they showed little attacking intent.
Ronaldo had another excellent chance to break the deadlock in the 59th minute but he sliced over after Giorgio Chiellini's cross was chested down by Bernardeschi.
Croatia striker Mandzukic then headed over, while left-back Alex Sandro smashed a left-footed drive narrowly wide.
The Bianconeri needed two goals in the final 20 minutes to win on this ground on their last visit back in February 2016 and they repeated the trick by breaking the hosts' resistance with 81 minutes played.
Miralem Pjanic attempted to unleash a shot from just inside the area and his blocked attempt broke kindly for Ronaldo to sweep a first-time effort into the bottom left corner.
The home side committed men forward in search of an equaliser and they were punished by a breakaway goal, led by Ronaldo.
He played the ball to Pjanic wide on the left and his cross was controlled by Bernardeschi, who fired the ball into the bottom right corner via the gloves of Sportiello.