Born in Harlem, Alexander is the son of Alethia Knox and Robert Whitehead. As a teen, he began dancing in New York clubs earning the nickname "Flex" due to his acrobatic dance skills. After being discovered by DJ Spinderella, he began touring with rap trio Salt-n-Pepa's dance troupe for three years. In addition to dancing, Alexander also choreographed for Mary J. Blige and Queen Latifah. In 1989, he made his stand-up comedy debut and eventually turned to acting, making his film debut in the 1992 film Juice. Alexander also was a regular cast member of the Sketch comedy show, Uptown Comedy Club, from 1992-1994.