It developed from the slave trade.
The earliest documented cases of Muslims to come to the United States were two West African slaves: Ayuba Suleiman Diallo, who was brought to America in 1731 and returned to Africa in 1734, and Omar Ibn Said in the mid 19th century.
There has been some speculation that perhaps a Moor slave Estevanico of Azamor who had converted to Christianity 14 years before his arrival in North America in the early 16th century, to have been at least the first born Muslim to enter the historical record in North America.
Once very small, the Muslim population of the US increased greatly in the twentieth century, with much of the growth driven by rising immigration and widespread conversion.
In 2005, more people from Islamic countries became legal permanent United States residents — nearly 96,000 — than in any year in the previous two decades.
Recent Immigrant Muslims make up the majority of the total Muslim population. Native-born American Muslims are mainly African Americans who make up a quarter of the total Muslim population. Many of these have converted to Islam during the last seventy years.
Conversion to Islam in prison, and in large urban areas has also contributed to its growth over the years. American Muslims come from various backgrounds, and are one of the most racially diverse religious group in the United States according to a 2009 Gallup poll.