Cameroon-born Mendomo, wearing a white bandana to keep his flowing dreadlocks in place, struck after just seven minutes with a 20-metre drive past goalkeeper Anas Zniti to settle a lively, close encounter.
Moghreb hopes of equalising suffered a blow just before the hour mark when defender Samir Zekroumi was sent off after a second yellow card.
The Moroccans failed to score on the road for the first time in five CAF Confederation Cup matches and the Tunisians remain title favourites ahead of the return match on 4 December in Fes.
Club Africain received a massive pre-match fillip when the Confederation of African Football lifted the second half of a two-match spectator ban meted out after crowd trouble during the group phase.
The crowd turned up to see hopes of an African double by Tunisian clubs increase after Esperance defeated Wydad Casablanca of Morocco 1-0 on aggregate last weekend to lift the CAF Champions League trophy.