In 1984, World Championship Wrestling (WCW) signed Magnum. After debuting, Magnum began feuding with Wahoo McDaniel and defeated him for the United States Heavyweight Championship on March 23, 1985. At The Great American Bash, he successfully defended the title by defeating Kamala. He then began feuding with the Four Horsemen before feuding solely with Horsemen member Tully Blanchard and his valet Baby Doll. On September 28, Magnum lost to the Horsemen's leader Ric Flair in a near-30-minute title match for Flair's World Heavyweight Championship. Magnum would later lose his U.S. Title to Blanchard until Magnum regained the title in a famous "I Quit" match at Starrcade on November 28
In April 1986, Magnum began feuding with Ivan Koloff, who had started proclaiming that his nephew, Nikita, would become the United States Heavyweight Champion. Jim Crockett, Jr. set up a contract signing for Magnum to defend his title against Nikita in May. At the signing, Magnum brought his mother, Marion, while Nikita brought Ivan. However, Nikita insulted Magnum's mother during the signing, instigating a fight. Then-president of WCW, Bob Geigel, issued a public reprimand against Magnum for "conduct unbecoming a champion." Magnum replied, "Reprimand this!" while punching Geigel. As a result, Magnum was stripped of his title and it was put up in a best of seven series between Magnum and Nikita. After losing the first three matches in a row to Nikita, Magnum began a comeback, winning the next three matches to keep the series going. In the tie-breaking match on August 17, 1986, in Charlotte, North Carolina, Nikita defeated Magnum to win the title with help from Ivan and Krusher Khruschev.
On October 14, 1986, Magnum was driving in the rain and lost control, crashing his car. The right side of his body was paralyzed for months, ending his in-ring career. Koloff gained respect for Magnum from their feud and wanted to take his place. Magnum's first on-screen appearance following the accident was on Supertowns on the Superstation, in an interview with Tony Schiavone. His first appearance in front of a live crowd was at the Crockett Cup in 1987, where, with the aid of a cane and two referees, Magnum walked to ringside to embrace Dusty Rhodes and Nikita, who would go on to defeat Tully Blanchard and Lex Luger for the tournament championship.
In his last angle in WCW in March 1988, Magnum was attacked by his former rival Tully Blanchard, which prompted Dusty Rhodes to come to the aid of his friend. Rhodes was subsequently suspended and returned as The Midnight Rider, managed by Magnum.
On June 24, 2007, Magnum T.A. made his first appearance for World Wrestling Entertainment at their Vengeance: Night of Champions pay-per-view, where he was noted as a former United States Champion.