Mane opened the scoring in the 10th minute at the Hong Kong Stadium with a powerful volley after a low cross from Croatian midfielder Marcelo Brozovic.
However, the Senegalese forward was shown a straight red card in the 25th minute for a reckless challenge on Al-Ittihad goalkeeper Hamed Al-Shanqity, confirmed after a VAR review.
The dismissal continued a troubling trend for Al-Nassr in the competition, following red cards for Cristiano Ronaldo in 2023 and Brozovic in 2024.
Dutch winger Steven Bergwijn equalised for Al-Ittihad in the 16th minute, finishing off a swift counterattack with a low shot from the edge of the box.
Despite being reduced to 10 men, Al-Nassr regained the lead in the 61st minute when Felix, who joined the club this summer, latched onto a through ball from Ronaldo and slotted past the keeper.