He debuted in WWF as Shawn Michaels bodyguard, a big dumb truck driver character named Diesel.
Riding the coattails of Shawn Michaels, they won Tag-Team Champions, and Nash was the Intercontinental Champion, but then Nash turned on HBK because he had a chance to jump up to the main event in a lucky turn of events.
Nash defeated Bob Backlund in 8 seconds to become WWF Champion. The match was on a non-televised house show... that shows you how much the WWF cared about him winning the belt.
Nash has admitted that the only reason he was brought up to the main event was because the whole Steroid trial was going on for Vince and he needed a "clean" big man. Vince only liked having giants as WWF Champion, so during the steroid trial he took the belts away from guys on steroids and tried to give them to "clean" wrestlers. The fans hated the whole Bob Backlund joke, and Vince obviously wanted a giant, but one that was "clean" and Nash was the only guy on the roster that fit the bill. So for the next year, Nash held the belt, despite never having a decent match!
After dropping the title, Nash decided to jump back to WCW because they offered him more money, and it would not require him to actually deliver in the ring at all. He just had to show up and they would pay him more than anyone in wrestling. If anyone got a better deal than him, WCW had to give him a raise lol.
After riding on the coattails of Shawn in WWF, Nash followed Scott Hall (Razor Ramon) to WCW and once there they joined forced with Hulk Hogan. So Nash continued to make his career just following real stars to the ring. He went from being Shawn's bodyguard, to Hogan and Halls. Jericho joked on Twitter than Nash was the 3rd wheel of the NWO... and its true.
Nash's highlight in WCW would have to be his Starcade main event with Goldberg when he ended the streak! Only he didn't do it with any wrestling talent, Scott Hall electrocuted Goldberg with a security guards stick, and Nash got the pin! It was possible the worst moment in WCW, they ended the best thing they ever had, and all for the ego of the worst wrestler they had. By this point, Hogan wasn't the top guy anymore, having past the torch to Goldberg, Hall had thrown his career away with his addictions, and Nash had no one left to follow. So this was his big chance to finally step up, and this was how he did it, what a shame.
Afterwards, WCW obviously had to take the belt off of Nash, but did they do it right? Did Nash do the honors, pass the torch, give a good match, put over the next Champ? No! Nash chose to drop the belt to Hogan. And they didn't even have a match... instead it was the infamous "Poke Of Doom" where Hogan poked Nash and he acted like he was shot and let Hogan get the pin. WHAT A JOKE! It was just the most pathetic moment in wrestling history in my opinion, it showed how much respect Nash really had for the business that had handed him everything his whole career. He never earned anything, so therefore he never respected anything.