The first African-American to speak in the US House of Representatives was a Presbyterian pastor!

Henry Highland Garnet (1815-1882) had been the pastor of the Fifteenth Street Presbyterian Church in Washington, D.C., for less than a year, when in 1865 the Chaplain of the US House of Representatives, William H. Channing, requested him to preach a memorial discourse on the occasion of the approval of the 13th Amendment to the US Constitution, abolishing slavery in the country. In so doing he became the first African-American man to speak in the US Capitol building. Garnet had been educated classically in New York, having escaped Southern slavery with his family when he was nine years old. He came into contact with the Presbyterian church through the ministry of Theodore Sedgwick Wright, and eventually became a Presbyterian pastor in New York, and then Washington, D.C. 

His life story and memorial discourse is found here, and is important reading for Presbyterians today.