The Portuguese forward scored a late close-range strike after his penalty had cancelled out Adrien Rabiot's opener at the end of a thrilling first half.
Brazilian left-back Marcelo wrapped up the victory minutes after Ronaldo's tap-in with a goal created by Marco Asensio.
The emphatic spot-kick meant Ronaldo became the first player in Champions League history to score 100 goals for the same club
Last season, Zinedine Zidane's Real became the first team to retain the Champions League and won La Liga for the first time since 2012.
This term they are already out of the Copa del Rey and fourth in La Liga, 17 points behind leaders Barcelona as pressure on their French manager has mounted.
PSG played their part in an end-to-end contest, with Neymar's delicate assist for Rabiot a highlight and Kylian Mbappe forcing a strong save from Real goalkeeper Keylor Navas.
But the result puts Real in control heading into the second leg at Parc des Princes on 6 March.
However, Zidane's side also survived a second-half penalty claim with the scores level when Rabiot's shot hit Sergio Ramos high on the arm.
Ronaldo not fading yet
Cristiano Ronaldo has 100 Champions League goals for Real Madrid
There will, inevitably, come a point when Ronaldo genuinely becomes a "fading star".
But once again tonight he proved that suggestions he is past his best are premature.
The 33-year-old may be losing the searing pace that has been one of his many attributes, but once again he showed he still rises to the big occasion like few others.
He has now scored 10-plus goals in seven consecutive Champions League campaigns, while no other player has done so in more than two. His last 12 appearances in the competition have seen him score 20 goals.
This contest was billed in some quarters as being all about five-time Ballon d'Or winner Ronaldo versus Neymar, the prodigiously gifted young contender.
But as a disgruntled Neymar headed straight down the tunnel at full-time, Ronaldo lingered to bask in the adulation of the Madridista faithful, from whom he had asked for "unconditional support" after striker Karim Benzema was booed during their 5-2 home win over Real Sociedad on Saturday.
If Ronaldo's penalty was dispatched with trademark power and precision, he got lucky with his second after the impressive Asensio's cross appeared to ricochet in off his knee.
But once again he was in the right place at the right time and with his talisman still firing, Zidane will remain hopeful of ending a disappointing season with glory in May.
Source: BBC Sports