If we are going strictly by a geographical definition, Egypt was conquered in 642 ad, during the reign of the caliph Umar. This was approximately ten years after the death of Muhammad.

If you mean something else by Africa, like "when did Islam reach sub-saharan Africa", the answer varies. Muslim merchants are believed to have reached the West African sahel in the 760s or 770s, associated with kharijite Berber dynasties in what's now Algeria.

There is also the hadith tradition that the Ethiopian Negus accepted early Muslims fleeing hostility in Mecca, even before Muhammad undertook the Hijra to Medina in 622 ad. However, I would not be confident in saying "this was the first spread of Islam to Africa" because of issues authenticating an oral tradition that is not codified until much later than the events they relate.