Ronaldo nearly got his goal midway through the second half, only for a deflected ball to bounce back off his heel just yards from the line to tee up Mandzukic to tuck home into an empty net.
Just as it was on Ronaldo's debut last week, when Juventus won 3-2 at Chievo Verona, it was down to his teammates to fire the Bianconeri to victory, with Miralem Pjanic also netting as the seven-time defending champions downed Lazio.
It meant that Ronaldo missed an open goal, with Mandzukic scoring from the rebound to double Juve's lead.
Ronaldo joined Juventus from Real in a £99.2million this summer, ending a remarkable nine-year spell with the Spanish side which yielded eight major trophies and five Ballon d'Ors.
"We went from being frenetic at times to falling asleep at others".
But that didn't stop Italian newspaper Tuttosport, which is based in Juventus' home city of Turin, from completely blowing Ronaldo's assist out of proportion.
Joao Cancelo's low cross from the right was partially intercepted by Lazio goalkeeper Thomas Strakosha but he only succeeded in pushing the ball onto Ronaldo's foot.
"Then everyone is waiting for his goal, but I'm happy with what he did and how he is settling in because he is a very humble guy".
The news was certainly a shot in the arm for fans of Lionel Messi, with whom Ronaldo is constantly compared and measured against in the battle for the "best player in the world" and "GOAT" tag. We'll find out in Juve's next game versus Parma on September 1.
"We know that if he scores goals it will help us win more matches". For him, for us, to understand his game.
In Saturday's second Serie A game, Ancelotti got his first home win as Napoli coach as the southerners fought back from two goals down to edge AC Milan.