Mahrez's free-kick sealed a 2-1 win at Parc des Princes in last week's first leg and he netted a goal in each half on Tuesday to close out a 4-1 aggregate triumph.
The former Leicester City favourite has found the going tough at times since joining City in 2018 but he is now a go-to pick for Guardiola, his four goals and two assists in this season's competition putting the club one game away from elusive Champions League glory.
"Riyad is an extraordinary player – big talent, huge quality, especially in the big games he is always ready," City manager Guardiola told a post-match news conference.
"We know his quality quite well. He is a fantastic player.
"The players in these stages are judged how they behave in the biggest scenarios, the big tests.
"He loves it and enjoys it. He made a good goal, the first one with right foot and the finish for the second. Three goals over the tie, he deserves big compliments.
Fernandinho was a surprise selection on his 36th birthday, but Guardiola felt the club captain excelled.
"Especially in the second half he was incredible," said Guardiola. "We adjusted a little bit [at half-time] to support John [Stones] and Ruben [Dias].
"He was so good, really, really good. It was his birthday, a big present. He has been an extraordinary captain this season, leading us in bad moments and he played really well."
Guardiola conceded it was strange to play such a key game behind closed doors, even if that has now been a reality of the coronavirus pandemic for almost a year.
Before the match, a group of City fans – some with flares – gathered to welcome the team bus to the ground.
"I have to say it was socially distanced but we loved it," Guardiola said. "It's nice to be in a final but we missed them."
"Of course, it was so weird playing the semi-final of the Champions League in empty stadium.
"The club belongs to the people, that's for sure. We are here for a period but the fans always stay. I am pretty sure they are so proud."