Assuming that you are asking about the time between the European "discovery" of the Americas and the late 1800s, Africans primarily came to the Americas as slaves.
Beginning in the early 1500s, Europeans were bringing Africans to the New World as slaves. As can be seen in the link below, there were 10,000 African slaves being brought just to the Caribbean (not counting Mexico or mainland South America) each year. According to some historians, the total number of slaves brought to the Americas was around 11 million by the time the slave trade ended.
During that time, then, most Africans who came to the Americas came as slaves.
I would add : they do not "come", but rather they were "brought" to America, after the original populations had been destroyed by the forced work.
Unfortunately, they came as slaves...