كتب : Ahmad Saied
Yamin Selemani scored twice for the winners and Henry Mbazumutima added the third while Buha Ibrahim registered Mauritania's only goal.
Despite the defeat, Mauritania still second with three points, trailing Egypt with one point and leading Burundi on goal difference.
Botswana, who managed to grab a historic goalless draw with African champions Egypt on Saturday, are in fourth spot with one point.