Heidenreich made his World Wrestling Entertainment debut on the September 29, 2003 episode of Raw with a gimmick being "controlled" by an entity known as "Little Johnny". After weeks of trying secure a tryout match, he was finally granted one on the October 27 episode of Raw, teaming with The Hurricane to defeat La Resistance. On the November 3 episode of Raw, he teamed with Trish Stratus to defeat Victoria and Stevie Richards. Over the next month, Heidenreich remained undefeated, before his streak was snapped by Rico on the December 14 edition of Sunday Night Heat. On the February 2, 2004 episode of Heat, Heidenreich was defeated by Rob Van Dam. After wrestling a few more times, Heidenreich disappeared from WWE television around May 2004.
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, which he referred to as "Disasterpieces." On the September 16 episode of SmackDown!, he attacked commentator Michael Cole and dragged him to the arena restroom to sexually assault him.
His first feud came against The Undertaker, and began after Heidenreich ran in during Undertaker's WWE Championship match against John "Bradshaw" Layfield 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 WWE Tribute to the Troops on December 23, Heidenreich lost to the Undertaker via countout. On the January 6, 2005 episode of SmackDown!, Heidenreich and Heyman lost to the Undertaker in a 2-on-1 handicap match, after Heyman was pinned. After the match, Heyman was placed in a casket by the Undertaker, writing him off television.
At Royal Rumble on January 30, he faced The Undertaker in a casket match. Midway through the match, Snitsky from the Raw brand interfered on Heidenreich's behalf. However, after opening the casket, 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 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; Undertaker instead fought Randy Orton and Kane was placed into the Money in the Bank ladder match. The feud between the four instead culminated at a live event on February 6, with Undertaker and Kane getting the win.
At No Way Out, Heidenreich lost to Booker T by disqualification after hitting Booker with a steel chair. On the March 3 episode of SmackDown!, Heidenreich faced Booker T in a rematch, which ended in a disqualification win for Heidenreich after Booker performed a DDT on a chair. Heidenreich and Booker faced once again on the March 10 episode of SmackDown! in a no disqualification match, which ended with Booker decisively defeating Heidenreich. On the March 17 episode of SmackDown!, Heidenreich read Booker a poem, thanking him for encouraging him to show the "real" Heidenreich, turning face in the process. Heidenreich participated in a 30-man battle royal at WrestleMania 21, which was won by Booker T.
His "disasterpieces" 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 stand in his corner during his match. He also had a brief feud with Orlando Jordan, unsuccessfully challenging him for the United States Championship at Judgment Day.
Heidenreich went on to feud with MNM (Melina, Johnny Nitro, and Joey Mercury) after they attacked him while he was eating chocolate with divas on the June 16 episode of SmackDown!. After being attacked week in and week out by the trio, he was finally helped out by Road Warrior Animal on the July 15 episode of SmackDown!. Animal and Heidenreich then challenged MNM to a match at the Great American Bash, where they defeated them to win the WWE Tag Team Championship.
On the July 28 episode of SmackDown!, Heidenreich reluctantly shaved his hair into a mohawk at Animal's request. After Animal convinced him he wasn't looking for a "replacement" for Road Warrior Hawk, 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. Heidenreich wrestled his final match, teaming up with Animal to defeat Nunzio and Vito, on the December 30 edition of Velocity.
On January 17, 2006, WWE announced that Heidenreich had been released from his contract.