Newly signed Senegalese forward Mane burst through the defence of the Iraqi champions before being brought down in the box after 75 minutes.
The referee initially waved away Al Nassr's appeals before VAR intervened to award the penalty, which Ronaldo duly stepped up to convert to give his side victory.
Former Liverpool star Mane, 31, joined Al Nassr this month from Bayern Munich on a four-year contract.
Al Nassr's other big summer recruits Marcelo Brozovic, Alex Telles and Seko Fofana also started the game at Prince Sultan Abdul Aziz Stadium in Abha, Saudi Arabia.
The nine-times Saudi champions, who finished runners up in the Pro League to Al Ittihad last season, dominated the game against Al Shorta, and Ronaldo thought he had scored early in the first half.
The Portuguese forward latched on to a through ball before firing past the goalkeeper only for his effort to be ruled offside.
Mane had spoken of his excitement over linking up with Ronaldo at an official launch event in Jeddah for the Saudi Pro League's new season.
Mane said: “I think you can see how quick everything goes for me, because he is one of the best players in history if not the best of course, and that was easy for me, he makes it easier for me.
"And (Brazil midfielder) Talisca and the other boys, they are all really great players and I am really excited to start the league with all these boys to make the club a real success.”
Mane spent six years at Liverpool before moving to Bayern in 2022. He played a key role in the 2018/19 Uefa Champions League win by Jurgen Klopp's side and then in capturing the Premier League title the following season. It marked the Anfield club’s first top-flight title in 30 years.
With 120 goals in 269 appearances following his 2016 transfer from Southampton, Mane also lifted the FA Cup, League Cup and Fifa Club World Cup with Liverpool.
He has tasted glory at international level, too, helping Senegal to the 2021 Africa Cup of Nations trophy. Mane has twice been named the continent’s player of the year, in 2019 and 2022.