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.
"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.