Miller made his World Championship Wrestling (WCW) debut when he saved Glacier from a two-on-one attack by Wrath and Mortis at the 1997 Slamboree pay-per-view. Initially thought as just a fan who had jumped the railing, the WCW commentators then realized that this was no ordinary man and he was in fact the three-time karate champion Ernest Miller. Glacier and Ernest Miller quickly became tag team partners in their feud with Mortis and Wrath. After Glacier turned on Miller, Ernest left him and became a heel with Sonny Onoo as his manager.
As a villain, Miller adopted an arrogant, trash-talking personality, calling himself "The Greatest" and gloating that he could beat anybody within two minutes, which often did using his signature kicks. It was also around this time that Ernest began wearing ruby slippers in the ring and dancing in a James Brown-esque fashion to celebrate his victories. He would claim on Nitro that he knew the actual James Brown, leading to a pay-per-view "dance-off" between Miller and Brown, who had been brought in. Miller even went as far promoting his own instructional, karate home video with a pseudo infomercial.
Miller then began a minor feud with Kaz Hayashi, as Onoo had previously been Hayashi's manager and had left him to manage Miller. Ernest won several matches over Hayashi with Onoo's help, and also enlisted Perry Saturn as his ally. However, Saturn suffered many losses against inferior opponents due to Miller's botched interference, so the two faced off at Starrcade, where Saturn won. Miller continued coming out and challenging the fans, the wrestlers, and anyone else to try and take him out, feuding with Scott Norton. For a time, he started coming out with the snow machine and music of Glacier, which Onoo had "purchased", but it was short-lived, and he soon returned to his James Brown music. In March 1999, Miller and Onoo attacked fellow martial artist Jerry Flynn, with Ernest cutting off Flynn's ponytail as a sign of disrespect. They all faced off in Uncensored in a handicap match, but Flynn defeated them both. Miller bounced back after some time, getting a win over Scott Norton after hitting him with a crowbar. He and Norton were scheduled to wrestle at The Great American Bash, but Norton was pulled out for health problems and replaced with Horace Hogan, who Miller defeated after kicking him with a steel-tipped red dancing shoe.
In June 1999, Miller had a dancing contest with Disco Inferno, who he attacked in jealously over the fans's attention. They fought at Bash at the Beach in a "dance match" with the stipulation that the loser could not dance anymore (though this stipulation was removed at Bash at the Beach), with Miller winning thanks to Onoo. The Cat's next opponent would be Buff Bagwell, who became famous by doing an impersonation of Miller with a bald cap and black face paint. The two met at Road Wild, but this time Onoo's help backfired when he accidentally hit the wrong man with a briefcase, making Miller lose the match. After that, the two broke up and Onoo left WCW.
Miller became a jobber for a long time, until he was chosen as Eric Bischoff's assistant. His role initially was composed of following Bischoff wherever he went and repeating whatever Bischoff said for comedic effect, but he was eventually appointed WCW Commissioner in 2000. As a commissioner, his motivations were the ratings and impressing the people in charge, and he often booked super matches for the main events. During this time, he got in a storyline where the Jung Dragons stalked him and tried to attack him, but Miller usually fended them off. In July 2000, Miller outlawed WCW United States Champion Scott Steiner's finisher, the Steiner Recliner, with the pretext of being too dangerous. Miller continued interfering in Steiner's reign, helping Mike Awesome during their fight at Bash at the Beach by disqualifying Steiner and stripping him of the title. Miller then feuded with yet another Japanese wrestler, The Great Muta, leading to a match at New Blood Rising. Although Muta used his Asian mist to blind Miller at one point, Ernest's ally Tygress interfered to hit Muta with a chair, which allowed Miller to land his finishing move for the victory.
After this match, he started to slowly turn into a fan favorite due to the high fan reaction he was getting whenever he entered an arena, eventually culminating to a rivalry with Mike Sanders from the Natural Born Thrillers stable, who sought to be the new commissioner. Miller offered Sanders a kickboxing match for the role, something which naturally had Mike afraid, but he agreed. During the match, Miller dominated three rounds, but during the last one he was attacked by Sanders' partner Shane Douglas, who pulled him out of the ring and counted him out. Angry at losing the commissioner title, Miller kept chasing Douglas for the next months, enlisting Miss Jones as his valet to counter Douglas' Torrie Wilson. Ernest and Douglas fought at Mayhem, where Miller won after Jones overpowered Wilson and gave Miller his old loaded red shoe, which he used to knock Shane out.
Miller then became an enemy of Lance Storm, the leader of Team Canada, with the two wrestling at Starrcade 2000. The former karate champion had to endure many interferences from Major Gunns and Elix Skipper, but although he and Miss Jones kept them at bay, she accidentally knocked the referee out with a misguided kick, which allowed a reluctant Hacksaw Jim Duggan to attack Miller, making him lose. Afterwards, Team Canada turned on Duggan and beat him down, until Miller saved him. Ernest turned again to get the commissioner's spot and challenged Sanders for it, but Mike wanted none of it, bringing in KroniK as mercenaries to take him down and put the role on the line only if Miller accepted to give him Miss Jones's services. Miller and Sanders fought at WCW Sin, where Sanders tried to hit The Cat with a chair, but Miss Jones impeded it. KroniK and the Natural Born Thrillers came to finally put Ernest away, but it was then revealed that KroniK had been secretly hired by Miller, which turned them against Sanders and made Miller win the match, regaining his place as the WCW commissioner.
Miller's victory was short-lived, as Lance Storm came to reclaim the commissioner role with Ric Flair and his Magnificent Seven's help. Storm managed to defeat Ernest for the job and used the commissioner role to aid his Team Canada, but Miller chased him and got a match for the role at SuperBrawl: Revenge, where he won with Miss Jones' aid, gaining it back. The karate champion then started a feud with Chris Kanyon, who attacked Miss Jones to the point it put her in the hospital. Miller even gave up his role as commissioner in order to get a shot at Kanyon, chasing him for months and even having a street fight in Jones' hospital room. Kanyon then attacked Miller's limo driver, M.I. Smooth. At the Greed pay-per-view, Miller and Kanyon faced, with The Cat having to protect Miss Jones during the match, but he was victorious with Jones and Smooth's help. Miller and his driver feuded with Kanyon and his ally, Road Warrior Animal, but the feud never had a conclusion due to the purchase of WCW by Vince McMahon in March 2001.
Under his Ernest "The Cat" Miller ring name and gimmick, he made his dark match debut for World Wrestling Entertainment on October 28, 2002. He then served as an announcer for Velocity from December 2002 until August 2003. After his run on Velocity, he became an active wrestler on the main roster, competing for the SmackDown brand.
Miller appeared in the 2004 Royal Rumble match, making his entrance (along with manager Lamont) while the other two people in the ring, Chris Benoit and Randy Orton, were knocked down. Miller and Lamont proceeded to dance to his theme music, not bothering to capitalize on the advantage he'd been given. When Benoit and Orton got back up, both men quickly tossed him from the ring. During his time, his catchphrase, "somebody call my momma" (which was also used in WCW), started to be used in WWE. This phrase also appeared in the lyrics to his theme song, which he would dance to. Miller then competed in a 15-man Royal Rumble on SmackDown! to earn a shot at the WWE Championship against then-champion Brock Lesnar at No Way Out, but was once again thrown out during his theme song, this time by Kurt Angle. Following this, he started a short-lived feud with Tajiri after Tajiri attacked Lamont with his trademark Buzzsaw Kick. After the feud ended, Miller was released from WWE on February 10, 2004.