Kevin Nash started out in the big leagues in WCW as Oz, a copy of the wizard from the Wizard of Oz. He had little success there, and then ended up in WWF in 1993 as Diesel, Shawn Michael's bodyguard. He soon started to wrestle and became tag champions with Michaels.
Michaels and Nash had a falling out and the two split. Nash won all three major titles, the Heavyweight, Intercontinental and Tag Team titles in the same calendar year - 1994 - the only wrestler to do so in the WWF, defeating Bob Backlund, Razor Ramon (Scott Hall) and the Headshrinkers, respectively. He even headlined WrestleMania XI against Shawn Michaels.
In 1995, he left the WWF and joined WCW with his friend Scott Hall as the Outsiders, who interrupted a night of Monday Nitro. The two later joined up with Hulk Hogan to form the nWo, a group of rebels who almost took over WCW. He won the Heavyweight title there a few times and reigned supreme until WCW was bought from WWF. While he was wrestling in the last days of WCW he was on his own as the nWo fizzled out a few years before.
In 2002 Kevin Nash joined back up with Scot Hall and Hulk Hogan to enter WWE as the nWo. They made a small impact, but Hulk Hogan left the nWo, Hall was fired from WWE and Nash injured himself. When he was fine to wrestle again, his first match back he re-injured his leg, so he was out for some time until 2003. He never made it back to the ring to wrestle because he was let go from the WWE. He has been in a few movies since.