The first African-American that served in the United States Senate was Hiram Rhodes Revels. Revels served as a US Senator for Mississippi from February 1870 until March 1871.
The reason Hiram Rhodes Revels served only 1 year as Senator is because he was elected to finish the term of one of the seats for Mississippi that had been left vacant since the American Civil War.
At the time, the US Senate seats were not elected by popular vote but were instead elected by the state legislature. The Mississippi legislature voted 81 to 15 electing Hiram Rhodes Revels to represent the state in Washington, D.C.
Despite being selected by the Mississippi legislature to represent the state in the US Senate, Revels’ fight to sit in office was not over. Some Democratic Party members of the Senate opposed the election of Hiram Rhodes Revels as he was black. Two days of debates ensued which ended on February 25th, 1871 in a vote on party lines of 48 to 8 in favor of allowing Revels to sit in the US Senate.
After his term in office, he was appointed as the first President of the historically black college, the Alcorn Agricultural and Mechanical College. In 1872-1873 Revels also briefly served as the Secretary of State of Mississippi. He also spent some time as a Methodist Episcopal minister. He died in 1901.
As well as being the first black United States Senator, he was also the first, and 1 of only 5 with avowed Native American ancestry to serve in the US Senate.
After Revels, there was another Black American US Senator for Mississippi elected in 1875, but after that, another African American did not serve in the United States Senate until Edward Brooke in 1967.