Van Horn boxed as an amateur for four years, winning Golden Gloves and Junior Olympic titles. With his father as his manager and trainer, he made his professional debut in New Orleans, Louisiana, on September 2, 1984. Van Horn was just five days short of his 16th birthday.
Since Van Horn was still a high school student when he turned professional, he was given the nickname "Schoolboy." In 1986, he moved to Lexington, Kentucky, to attend the University of Kentucky. Boxing with the letters "UK" on his trunks, the "Schoolboy" angle was played up heavily.
After five consecutive wins, Van Horn had a rematch with Rosi in Italy on July 21, 1990. Although Van Horn fought better than he did in the first fight against Rosi, he was unable to regain the title. Rosi once again won by a 12-round unanimous decision.
Van Horn moved up in weight and won the IBF Super Middleweight Championship with an 11th-round knockout of Lindell Holmes in Italy on May 18, 1991. In his first title defense, he scored a third-round knockout of mandatory challenger John Jarvis in Irvine, California, on August 17, 1991.
On January 10, 1992, Van Horn lost the title to Iran Barkley, the former WBC Middleweight Champion, by a second-round technical knockout in New York City. Barkley, a 2-1 underdog, wobbled Van Horn with a left hook early in the first round and floored him three times in the second.
In May 1992, Van Horn graduated from the University of Kentucky with a bachelor's degree in broadcast journalism.
Van Horn was scheduled to face James Toney for the IBF Super Middleweight Championship in Tulsa, Oklahoma, on October 29, 1993, but he pulled out of the fight after claiming a shoulder injury. The Boston Globe reported: "Sadly, word around boxing says his real problem has been near-constant headaches that recently forced him to stay in a darkened room for days at a time." The Van Horn camp denied that was true, but one fight figure was quoted as saying: "It would be child abuse for his father to put him back in the ring. When I was with him, he knew me and why I was there, but every 15 or 20 minutes he'd ask, 'Why are you guys here?' It's pretty sad."
Van Horn was scheduled to face Nigel Benn for the WBC Super Middleweight Championship in England on September 10, 1994, but the fight was called off. According to Boxing Monthly, it was cancelled after Van Horn failed a brain scan. However, during an interview with Boxing News in 2015, Van Horn said: "I never failed any scan, not ever. I have no idea where that came from. I fought a few times after the Barkley fight. Rumors and things come up, it's just ridiculous. I just became disenchanted with the sport. I said to myself I was going to step back for a while and get a few things done; like going back to college, and my intention was to come back to boxing. But I never did."