Battling a thigh injury, Mbappe came off the bench with his team trailing 2-0, forcing a Holmar Eyjolfsson own goal before converting a 90th-minute penalty to salvage a 2-2 draw in the friendly in Guingamp.
Lloris was full of praise for the 19-year-old, who has made an impressive start to the season with Paris Saint-Germain, scoring 10 goals in seven games.
"Football is not only about one player but yes, he has been decisive. He gave us a lot of energy when he came on," he said.
"He was involved with the two goals so yes, he has been decisive, but there are also other players who are working a lot.
"Everything wasn't perfect. There were some good and bad points, now this game has to get us ready for the next game on Tuesday [against Germany in the Nations League] which is much more important."
Birkir Bjarnason and Kari Arnason had given Iceland their two-goal lead before Mbappe was introduced on the hour-mark.
Olivier Giroud, who played 90 minutes, also lauded Mbappe, and he is hoping for improvement from his team against Germany.
"Kylian has been a good sub, he was powerful. That's what we needed against this kind of team – a player who knows how to make the difference," he said.
"He does it really well, we all know what he is capable of. He got us out of a tight spot, it's good for him. It shows he is in good form at the moment.
"Anyway, I hope we will react better on Tuesday."
Didier Deschamps also priased Mbappe's effort while adding the young forward will be fit to face Germany in France's next match.