He moved to WCW in 1989 as Mean Mark Callous, teaming with Dan Spivey and replacing Sid Vicious in the Skyscrapers. Callous defeated Brian Pillman at the Clash of Champions in 1990 and lost to Lex Luger at the 1990 Great American Bash. 

    Callous signed with the WWF in the late summer of 1990 and began working house shows as "Kane the Undertaker" with Brother Love as his manager. There was a period where it was rumoured he would be the person coming out of the giant Egg at Survivor SEries 1990, but instead he joined with Ted DiBiase, Honkey Tonk Man and Greg Valentine as the mystery partner to defeat Dusty Rhodes, the Hart Foundation and Koko B. Ware. The Undertaker went on an amazing winning streak, appeared in the 1991 Royal Rumble and then switched managers as Brother Love left the WWF and Paul Bearer entered. At Wrestlemania VII, the Undertaker defeated Jimmy Snuka. The next month, Paul Bearer wheeled a casket down to ringside during a Sgt. Slaughter/Ultimate Warrior match and the Undertaker popped out to attack the Warrior and lock him in the casket. WWF Champion Hulk Hogan ran out to save the day but his blows had no effect as the Undertaker proved impervious to pain, teasing a future match between the two. 

    The Undertaker spent the summer feuding with Sid Justice, Randy Savage, Elizabeth and Hulk Hogan, while teaming with Jake Roberts and Ric Flair, before defeating Hogan at Survivor Series 1991 for the WWF Title. He lost the gold a few days later at This Tuesday in Texas on PPV. The World Title was held up in the 1992 Royal Rumble, where the Undertaker was eliminated by Hogan. 

    The Undertaker surprised many by turning face to feud with Jake Roberts, whom he defeated at Wrestlemania VIII. The Undertaker continued to feud with rule breakers, including Kamala, Nailz, Mr. Hughes, the Berzerker, and the Giant Gonzalez, who he defeated at Wrestlemania IX by disqualification and decisively at Summer Slam 1993. The Undertaker replaced Tatanka in the 1993 Survivor Series to team with Lex Luger and the Steiner Brothers against Yokozuna, Ludwig Borga, Jacques Rougeau and Crush, but was eliminated in a double count out with Yokozuna, beginning a feud that climaxed in a Casket Match, which the Undertaker lost after interferance from over a dozen "rule breakers", at the 1994 Royal Rumble. After the loss, the lights began to flicker, the video wall had a picture of the Undertaker and a very theatrical exit was made with the casket oozing green smoke, bringing on the era of the mystical character of the Undertaker, with increasingly theatrical spots that would carry on throughout his career. The Undertaker took some time off, but in the summer of 1994, Ted DiBiase brought out an imposter Undertaker and the real Undertaker returned at Summer Slam 1994 to meet and defeat the fake Undertaker with the tombstone. The Undertaker also got revenge on Yokozuna, defeating him at Survivor Series 1994 in a Casket re-match. The Undertaker began feuding with Ted DiBiase's Million Dollar Corporation, defeating King Kong Bundy at Wrestlemania XI and defeating Kama, Nikolai Volkoff and IRS in pay per view matches. In 1995, the Undertaker began feuding with King Mabel and began to mill around the World Title scene. In January of 1996, the Undertaker made it known that he wanted the World Title and beat Bret Hart by disqualification when Diesel interferes at the 1996 Royal Rumble. The Undertaker and Diesel would engage in a heated feud, ending in the Undertaker defeating Diesel at Wrestlemania XII. Shortly after, Mick Foley debuted as Mankind and began a very interesting feud that saw Paul Bearer switch allegiances to manage Mankind and ally with Goldust. The feud was to end in a Buried Alive Match but when the Undertaker lost, a lightning bolt hit the grave site and the hand of the Undertaker came out, setting the stage for an even more demonic Undertaker return at Survivor Series 1996 where he defeated Mankind. The Undertaker would almost capture the WWF Title in a Final Four Match in February of 1997, but then captured it by defeating Sid at Wrestlemania XIII. The Undertaker's title run would see him blackmailed by Paul Bearer, who claimed he had a secret that would destroy the Undertaker. We eventually found out the secret was that the Undertaker had killed his family, including his baby brother. On an edition of RAW, we learned that Kane was alive and the Big Red Machine debuted to wage war on the Undertaker, climaxing in a match won by the Undertaker at Wrestlemania XIV. 

    The Undertaker would turn heel and bring back Paul Bearer as his manager and teaming with Kane to attack the new WWF CHampion, Steve Austin. During this time, the Undertaker forged the Ministry of Darkness, an occult like group including the Acolytes, Bearer, members of the Brood, Viscera and Mideon. The alliance with Vince McMahon's Corporation ended with a heel versus heel feud where the Undertaker beat the Big bossman at Wrestlemania XV in Hell in a Cell. The Undertaker took time off to heal an injury and returned without the dead-man gimmick... but as a biker: The American Bad Ass era began. He was a fan favourite in 2000, feuding with Triple-H, the McMahons, Chyna and others. He defeated Triple-H at Wrestlemania XVII in his 10th straight Wrestlemania victory, before teaming with Kane to win the WWF World Tag Team Titles, which they lost to Triple-H and Steve Austin 10 days later. When WCW was purchased and the Invasion angle began, the Undertaker was the inspirational leader of the WWF and feuded with Dallas Page, Kronic and other insurgents before captaining team WWF to defeat ECW/WCW at Survivor Series. The Undertaker would turn heel to feud with Ric Flair and defeat him at Wrestlemania XVIII. He became the Hardcore Champion during this time, as well. The Undertaker defeated Hulk Hogan at Judgment Day 2002 to become the undisputed WWE Champion and had classic matches with Kurt Angle, Jeff hardy and the Rock. The Undertaker would leave and then return again at the Royal Rumble, leading to Wrestlemania XIX where the Undertaker defeated Big Show and A-Train in a handicap match. The Undertaker would lose a Buried Alive Match to Vince McMahon at Survivor SEries when Kane interfered. 

    The Undertaker made another comeback, reprising his Dead Man gimmick at Wrestlemania XX complete with Paul Bearer, to defeat Kane. The Undertaker would feud with Smackdown General Manager Paul Heyman, the Dudley Boys and Booker T. Heyman would take Bearer hostage and blackmail the Undertaker to do evil deeds, leading to a "Live or Die" match at the Great American Bash where the Undertaker defeated the Dudleys, then pulled the switch to "kill" Paul Bearer.  The Undertaker continues to wrestle in a less active role, wrestling less and "appearing" more with the theatrical gimmick of the Dead Man. He sent Mohammud Hassan out of the WWE when UPN pulled the plug on the gimmick. He defeated Randy Orton at Wrestlemania XXI and then defeated Mark Henry at Wrestlemania XXII in a Casket Match. He remains a perennial contender to any World Title and his longevity and impact has made him a truly worthy member of the UWOW Hall of Fame.