Seales made his breakthrough in 1978 with the PBS drama, Trial of the Moke, portraying Lt. Henry O. Flipper, the first African-American graduate of West Point. He went on to appear in The Onion Field (1979), in which he portrayed real-life convicted cop killer Jimmy Lee (Youngblood) Smith. That same year, he also had a minor role in Star Trek: The Motion Picture. He appeared in the 1981 film, Southern Comfort, in which he portrayed Rifleman Cleotis Simms.