The following is excerpted from Islam in the United States on Wikipedia:
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. The new position has been created under white house executive office as a United States special envoy to the Organization of the Islamic Conference to promote relation between Islamic world and United States government.
Recent immigrant Muslims make up the majority of the total Muslim population. South Asians Muslims from India and Pakistan and Arabs make up the biggest group of Muslims in America at 60-65% of the 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.
Muslim Americans are racially diverse communities in the United States, two-thirds are foreign-born. The majority, about three-fifths of Muslim Americans are of South Asian and Arab origin, a quarter of the population are indigenous African Americans, while the remaining are other ethnic groups which includes Turks, Iranians, Bosnians, Malays, Indonesians, West Africans, Somalis, Kenyans...
In 2005, according to the New York Times, more people from Muslim countries became legal permanent United States residents — nearly 96,000 — than in any year in the previous two decades. In addition to immigration, the state, federal and local prisons of the United States may be a contributor to the growth of Islam in the country. J. Michael Waller claims that Muslim inmates comprise 17-20% of the prison population, or roughly 350,000 inmates in 2003. He also claims that 80% of the prisoners who "find faith" while in prison convert to Islam. These converted inmates are mostly African American, with a small but growing Hispanic minority.
The following is excerpted from Muslim Americans (a PDF document):
The U.S. Census Bureau, as a matter of policy, does not ask about a respondent’s religion in the decennial census, the yearly American Community Surveys, or its monthly Current Population surveys. In addition, U.S. Citizenship and Immigration Services does not ask the religion of immigrants or naturalized citizens, leaving researchers to extrapolate the size of the population from information about nationality and language.
Most U.S. Muslims (65%) are first-generation immigrants. But more than a third (35%) were born in the United States. One-fifth (21%) of the native-born (or 7% of all Muslims in this country) are second generation, with one or both parents having been born outside of the U.S. The nearly two-thirds who were born outside of the United States come from at least 68 different nations, with no single nation accounting for more than 12% of the immigrants. More than a third (37%) of all foreign-born Muslim Americans arrived from the Arab region, including Arabic-speaking countries in the Middle East and North Africa. An additional 27% emigrated from the South Asian region, including Pakistan, India, Bangladesh and Afghanistan. Another 8% come from European countries and 6% from other parts of Africa. In terms of specific countries, 12% of foreign-born Muslims arrived from Pakistan, and the same proportion from Iran. No more than 7% of first-generation immigrants were born in any other single country. A majority of the foreign-born arrived in the U.S. in the 1990s (33%) or in this decade (28%). An additional 23% came during the 1980s, while just 16% came earlier than that.