Ablie Jallow’s superb strike gave them the lead early in the first-half, and that proved to be enough for the three points as they made a perfect start to Group F.
With the top two in each group progressing, as well as the four best third-placed teams, Gambia are well-placed to make it into the knockout stages on their tournament debut.
Kick-off was delayed for 45 minutes, following the farcical scenes in the earlier Group F match as Mali returned to the pitch to play the last few minutes of stoppage time after a refereeing error, but Tunisia refused in protest.
There was then further confusion as attempts to play the Mauritania national anthem were unsuccessful, but when the action did get underway it was Gambia who started the better of the side.
Jallow gave his side the lead in the tenth minute, as he took one touch before firing a brilliant effort right into the corner of the net from just outside of the box.
Mauritania grew into it from that moment and they had much the better of the first-half despite the scoreline.
Aboubakar Kamara blasted over from a tight angle, before Pape Ba had a shot blocked and a header tipped over the bar.
Gambia were bright after the break and had a number of chances to double their lead.
Sulayman Marreh and Omar Colley both headed over when completely free from corners, while Ebrima Colley’s left-footed strike was wayward after he worked the space inside the box.
Kamara had a shot comfortably saved as Mauritania tried to pile the pressure on as they searched for an equaliser. Idrissa Thiam claimed his free-kick was blocked by a hand, but after a long VAR check no penalty was given.
Gambia looked the most likely to score late on though as they threatened on the break, and they stood firm to secure the three points.