The goal was Ronaldo's 55th from a free kick during his illustrious career and also made him the first player in the Premier League era to score 25 or more goals in a season in England, Spain and Italy.
Buffon, 42, overtook Paolo Maldini's Serie A appearance record when he was picked for his first league game this year.
Torino responded well and forced a flurry of corners only to be caught out on the counter in the 29th minute when Ronaldo burst forward and slipped the ball to Juan Cuadrado. The Colombian outfoxed his marker and scored with a low shot into the far corner.
The visitors replied with a penalty given for handball against Matthijs De Ligt after a long VAR review in first-half stoppage time and Andrea Belotti beat Buffon to score for the fourth-successive game.
Torino were threatening an equaliser until Ronaldo, who became the first player to score 25 goals in a season for Juve since 1961, curled his shot over the wall and into the corner. It was his first goal in 43 free kick attempts during just under two seasons with Juventus.
"We took the lead right away and then scored again. Maybe we thought it was going to be easy but the penalty got us nervous and made us lose control of the game," said Juventus coach Maurizio Sarri.
"We had a confusing phase at the start of the second half and then, when the two teams were tired, our quality came out."
Torino had a goal ruled out for offside after the restart and Simone Verdi tested Buffon with a long-range shot but Torino's hopes of a fightback were ended by Ronaldo's 25th league goal of the season.
After the match, Ronaldo said: "We knew it wasn't going to be easy, but we deserved to win, we played well.
"My goal? I was waiting to score on a free kick. I'm happy, but it's important to win regardless of who scores. We are improving as a team, that's what matters."
The win left Juventus with 75 points from 30 games atop the Serie A table.