Horn earned a RBWI black belt under Bussey in 1992, and spent most of his career with the Miletich Fighting Systems team in Iowa.

Horn made his next appearance for the organization at UFC Brazil five fights later against Brazilian Vale Tudo fighter, Ebenezer Fontes Braga.

His 18-fight winning-streak was snapped in his next fight after suffering a loss via majority decision at the hands of Hiromitsu Kanehara in Horn's RINGS debut in Tokyo, Japan.

Horn then followed this up with wins over Kristof Midoux, Travis Fulton, Chalid Arrab, and Homer Moore before fighting for the King of the Cage Light Heavyweight Championship against then-champion, Vernon White, who had won and defended the title four times.

Two fights later, Horn fought in the IFC Light Heavyweight tournament on September 6, 2003, and won his first bout of the night via armbar submission before facing future Ultimate Fighter Winner, UFC Light Heavyweight Champion, and UFC Hall of Famer, Forrest Griffin in a highly anticipated bout.

Horn's next fight of the night was for the final against future Strikeforce Light Heavyweight Champion Renato "Babalu" Sobral.

At the start of the second round, the two exchanged submission attempts until Horn, while ground and pounding Sobral from the top, unintentionally landed an illegal knee strike to the face of the downed Muay Thai fighter.

Two fights later, Horn fought for TKO Major League MMA against UFC veteran, David Losieau and won via guillotine choke submission under one minute into the first round.

Horn then fought against future UFC veteran Chael Sonnen and won via TKO due to a cut 3:34 into the fight.

Two fights after the win over Sonnen, Horn fought future longtime UFC Middleweight Champion, Anderson Silva in Seoul, South Korea.

[4] At a September 9, 2006, International Fight League show, Horn lost to former Olympic wrestling silver medalist Matt Lindland in the second round due to strikes.

Horn's next bout, the 100th of his career, was a loss against future Strikeforce Middleweight Grand Prix Champion Jorge Santiago, but then won his next bout via unanimous decision against future Ultimate Fighter winner and current UFC veteran, Court McGee, who had been undefeated heading into the fight.

After two consecutive losses, Horn faced Brazilian jiu-jitsu specialist Rousimar Palhares at UFC 93 in Dublin, Ireland and lost again via unanimous decision.

[4] After leaving the UFC, Horn then fought Sean Salmon and defeated him in the first round by rear-naked choke to win the IFC Middleweight Championship.