Heidenreich made his World Wrestling Entertainment debut on the September 29, 2003 episode of Raw with a gimmick that had him first trying to secure a tryout on the different WWE brands, then being "controlled" by an entity known as "Little Johnny". Despite speculation as to Little Johnny's identity – Heidenreich's doll, a doll similar to George "The Animal" Steele's "Mine", a split personality – the angle was dropped before being resolved.
He returned on the August 26, 2004 episode of SmackDown! as a heel under the management of Paul Heyman. His new gimmick was as a psychopath: running in during random matches, attacking fans, and reciting hateful poetry. On an episode of SmackDown!, he attacked commentator Michael Cole and dragged him to the arena restroom to frighten him.
His first feud came against The Undertaker, and began after Heidenreich ran in during Undertaker's WWE Championship match at No Mercy. Undertaker beat Heidenreich at the next month's Survivor Series, but again Heidenreich cost him a WWE Championship match at December's Armageddon when he interfered in the main event fatal four way match not once, but twice.
At Survivor Series and at the 2005 Royal Rumble, Heidenreich had encounters with like-minded psychopath from the Raw brand, Snitsky. During their brief Survivor Series meeting they exchanged their views on each other – while breathing heavily, a mannerism they shared, Snitsky told Heidenreich "I like... your...poetry."; and Heidenreich replied "I...like what you do...to babies", referencing an earlier Raw angle that had Snitsky accidentally cause the miscarriage of Lita's baby.
At the Royal Rumble, Heidenreich broke out on his own; he secretly partnered with Snitsky, who agreed to interfere in Heidenreich's Casket Match with The Undertaker. During the match it was revealed that Kane was hidden in the casket, who pounced on both Snitsky and Heidenreich; they continued their fight in the crowd, while Heidenreich lost the Casket Match. This started a chain reaction of feuds, originally planned to lead to a match at WrestleMania 21 in which The Undertaker would partner with Kane to take on Snitsky and Heidenreich. This idea was later nixed by The Undertaker, who instead fought Randy Orton.
After a short feud with Booker T, Heidenreich turned face, altering his psychotic gimmick to become a more sympathetic character. Calling them "disasterpieces", his poetry became more light-hearted and began receiving cheers from the crowd. Heidenreich was then given a segment on SmackDown! in which he "made friends" with audience members, read them a piece of his poetry, and had them to stand in his corner during his match. He also had a brief feud with Orlando Jordan, unsuccessfully challenging him for the United States Championship.
Heidenreich went on to feud with MNM (Melina, Johnny Nitro, and Joey Mercury) after they attacked him while he was eating chocolate with the divas, during an event in Hershey, Pennsylvania. After being attacked week in and week out on SmackDown!, he was finally helped out by Road Warrior Animal. Animal and Heidenreich then challenged MNM to a match at the Great American Bash, where they defeated them to win the Tag Team Championship.
On the July 28, 2005 episode of SmackDown!, Heidenreich shaved his hair into a mohawk at Animal's request with reluctance. After Animal convinced him he wasn't looking for a "replacement" for Road Warrior Hawk to become Road Warrior Heidenreich, but simply a partner with talent, Animal talked him into wearing face paint and, on the August 18 SmackDown!, he was made an "official" member of the Legion of Doom and presented with a pair of Road Warrior spikes. On the October 28 episode of SmackDown!, the Road Warriors lost the Tag Team Championship back to MNM in a match also involving The Mexicools and William Regal and Paul Burchill.
On January 17, 2006, WWE announced that Heidenreich had been released from his contract.