No, the United States has never had an African-American president before. I honestly did not know if I might see it in my lifetime, and I am only 38 years old. Martin Luther King, Jr., was a very important civil rights activist and preacher who forever changed the civil rights world for the better. His contributions are endless and timeless.
We very well may have our first president who is half African-American (his father was African) and half-Caucasian (his mother was white). Many people refer to Barack Obama as African-American; however, he is half-African American and half-Caucasian. Mr. Obama is very proud of his mixed heritage.
One of the things that makes me very excited that he may become president is that so many older African-Americans who never thought they would see an African-American be president will finally have that chance (hopefully). It has been amazing to be a witness to the historic nature of Obama's rise to prominence. Incredible!
While the United States has never had an African-American president, many leaders from the black community have aspired to hold the office.
As recently as the 1980s, Jesse Jackson ran unsuccessfully for president during the Reagan era. Jackson and similar leaders such as Al Sharpton and Kwaze Mfume (check that spelling) act more as social watchdogs than as national representatives.
While MLK never became president, many credit him with the successes of the civil rights era, and cite him as the motivator of today's minority and majority politicians.
Neither Martin Luther King or Martin Luther King, Jr. were United States Presidents.
The attached link will give you a list of all the USA's presidents and a biography of them.