LeRoux debuted in World Championship Wrestling (WCW) in July 1998, and wrestled on WCW Saturday Night, WCW WorldWide and WCW Thunder as Mark LeRoux for several months, often losing squash matches.
LeRoux debuted on Nitro on February 2, 1999 as Lash LeRoux, losing to then-Cruiserweight Champion Billy Kidman. LeRoux competed primarily in the cruiserweight division. He feuded with Disco Inferno over the Cruiserweight Championship, and lost to Disco in a title match at Halloween Havoc 1999. Shortly afterwards, LeRoux and Disco joined forces against Tony Marinara and his henchmen, The Mamalukes, who were owed money by the compulsive gambler Disco. They faced Big Vito and Johnny the Bull in a tag team match at Starrcade 1999, but lost following a miscommunication between the partners. After the match, the unconscious Disco Inferno was carried from the arena inside a body bag and locked in the trunk of a car.
He appeared sporadically on television, teaming with Midnight and pursuing the Cruiserweight Championship once more. He was fired in storyline by WCW President Eric Bischoff in April 2000 for not helping to protect him from Diamond Dallas Page, whose wife Kimberly had betrayed him in order to align herself with Bischoff.
LeRoux and several other wrestlers who had been fired by the disgruntled President returned several weeks later as the Misfits In Action (M.I.A.), a military-themed stable who were unwilling to align themselves with either the heel New Blood or the face Millionaire's Club. All of the Misfits were given ranks, with LeRoux known as Corporal Cajun.
The M.I.A. feuded with the New Blood, with LeRoux helping Lieutenant Loco retain his Cruiserweight Championship. LeRoux and Loco won the WCW World Tag Team Championship on October 9, 2000 in Sydney, Australia, defeating New Blood members Mark Jindrak and Sean O'Haire, but lost the belts just minutes later in an instant rematch ordered by commissioner Mike Sanders (also a New Blood member). The M.I.A. also feuded with Team Canada, who had taken possession of the United States Heavyweight Championship and renamed it the "Canadian Heavyweight Championship", during which time M.I.A.'s valet, Major Gunns, turned on them and joined Team Canada.
The M.I.A. gradually disbanded in late 2000 and early 2001, with the departure of Loco and Sergeant A.W.O.L. precipitating the end of the stable. On January 23, 2001, the leader of the M.I.A., General Rection, requested LeRoux's M.I.A. shirt, informing him that he had been "honorably discharged". LeRoux began wrestling as Lash LeRoux once more, and returned to the cruiserweight division. His last appearance on WCW television was on the February 19 episode of Nitro, where he challenged United States Champion Rick Steiner and was easily defeated.