Mauritania's Red Crescent organisation said on Monday that the two separate incidents took place on Saturday and Sunday in waters north of the Mauritanian coastal city of Nouadhibou.
Ahmedou Ould Haye, a Red Crescent spokesman, said more than 40 of the migrants, most of whom came from Mali, Guinea Bissau and Gambia, were rescued.
Others came from Ivory Coast, Nigeria and Mauritania.
In the first incident, an open boat carrying 43 migrants broke in two and sank in rough seas. It had collided with a Moroccan vessel which had come to its assistance, Haye said.
Twenty-three people drowned and the 20 survivors were brought to Nouadhibou, a fishing port at the extreme north of Mauritania's Atlantic coast.
In the second incident, another open boat carrying 45 migrants capsized and 22 drowned. Mauritanian authorities rescued the survivors.
Thousands of Africans leave their homelands every year on a hazardous journey by land and sea in the hope of a better life in Europe or elsewhere. But many drown or die in the attempt.
Migration has become an even more sensitive issue since a wave of African migrants tried to force their way in September into the Spanish enclaves of Ceuta and Melilla on Morocco's northern coast. Several were killed.