Final answer:
Jerry Rice is the famous Mississippi Valley State University alumnus who had a 20-season NFL career, mainly with the San Francisco 49ers.
Step-by-step explanation:
The famous football player who is an alumnus of Mississippi Valley State University and played in the NFL for 20 seasons primarily with the San Francisco 49ers is Jerry Rice. This notable individual's career is an inspiring example of excellence in the sport, rising from a historically black university to become a football legend. His journey echoes the challenges and achievements of many African American athletes who have navigated both their racial identity and their athletic ambitions, as the excerpt about the struggle of being a Black athlete in a predominantly white environment describes.