The defending champions were expected to make a winning start to their Group E campaign and, whilst Mahrez went close to breaking the deadlock, Sierra Leone’s spirited performance was enough to earn them a point in their first game in the competition since 1996.
The Manchester City winger captained the side and played the full 90 minutes in Douala, Cameroon, where he was at the heart of Algeria’s best play.
It was the Leone Stars who started the brighter, with Alhaji Kamara sending the two best chances of the first half wide, whilst at the other end, Islam Slimani fired into the side netting from a tight angle after a precise through ball from Mahrez.
Algeria, who are coached by former City midfielder Djamel Belmadi, dominated the second half but were unable to find a breakthrough despite creating a host of chances.
Mahez was twice denied by goalkeeper Mohamed Nbalie Kamara, who got down well to turn an angled drive round the post and made a crucial stop on the edge of his area to keep out a curling 25-yard effort following his weak headed clearance.
Nbalie Kamara made important saves from Ramy Bensebaini and Baghdad Bounedjah in stoppage time and, whilst he enjoyed a fine afternoon between the posts, Algeria were also guilty of wasting several promising chances.
Yacini Brahimi scuffed a shot straight at the goalkeeper after a clever chipped pass from Mahrez and Said Benrahama dragged a golden opportunity wide with five minutes remaining as the Desert Foxes were forced to settle for a point.
The draw means Algeria extend their unbeaten run to 35 senior internationals, moving within two games of Italy’s world record 37 matches without defeat.
Their tournament continues on Sunday, when they face Equatorial Guinea in their second group stage game.
Kick-off is at 19:00 (UK) and we’ll bring you the latest news from that game on mancity.com and the official app.