When Portuguese explorers reached East Africa in 1498, Swahili and Arab commercial and slave trading settlements had existed along the coast and outlying islands for several centuries. From about 1500, Portuguese trading posts and forts became regular ports of call on the new route to the east. The voyage of Vasco da Gama around the Cape of Good Hope into the Indian Ocean in 1498 marked the Portuguese entry into trade, politics, and society in the Indian Ocean world. The Portuguese gained control of the Island of Mozambique and the port city of Sofala in the early 16th century, and by the 1530s small groups of Portuguese traders and prospectors penetrated the interior regions seeking gold, where they set up garrisons and trading posts at Sena and Tete on the Zambezi River and tried to gain exclusive control over the gold trade. The Portuguese attempted to legitimate and consolidate their trade and settlement positions through the creation of prazos (land grants) tied to Portuguese settlement and administration. While prazos were originally developed to be held by Portuguese, through intermarriage they became African Portuguese or African Indian centres defended by large African slave armies known as Chikunda. Historically within Mozambique there was slavery. Human beings were bought and sold by African tribal chiefs, Arab traders, and the Portuguese. Many Mozambican slaves were supplied by tribal chiefs who raided warring tribes and sold their captives to the prazeiros.
Muller started his career as a businessman, trading with East and West Africa. In his mid-twenties he travelled to Zanzibar, Mozambique, and South Africa for business purposes, but showed himself a keen ethnographer as well, collecting ethnographic artifacts and writing reports about the societies and people he encountered on his way. In 1890, Muller retired from business for personal reasons, and went to Germany to study ethnography and geography. He graduated with a Ph.D. dissertation four years later.