In June 1999, Yang signed with World Championship Wrestling, making his WCW television debut in January 2000 under the ring name Jimmy Yang. He was originally a part of The Jung Dragons faction along with Jamie-San and Kaz Hayashi. They were brought into WCW by Jimmy Hart to feud with 3 Count, another faction. They made their pay-per-view debut at New Blood Rising, losing a ladder match to 3 Count. They added a manager, Leia Meow, and continued the feud with 3 Count, until, in late 2000, Jamie-San unmasked and left the group, forming a tag team with former 3 Count member Evan Karagias. The team of Hayashi and Yang feuded with Karagias and Jamie until the WWF purchased WCW.
In September 2003, following a match with Tommy Dreamer on Sunday Night Heat, Yang was signed to a deal with the renamed World Wrestling Entertainment (WWE). He made his WWE debut on October 19, 2003 at No Mercy, along with Ryan Sakoda. Yang was renamed Akio and aligned himself with Sakoda, forming the villainous tag team Kyo Dai, the henchmen of the native Japanese wrestler Tajiri. Sakoda was eventually released from WWE and Tajiri was traded to Raw. This left Akio on his own to become a lower card wrestler and Velocity mainstay, competing primarily in the cruiserweight division until he was released on July 5, 2005.
In May 2006, Akio made a return to WWE on an edition of Heat, working as an enhancement talent in a try-out match with Charlie Haas. Beginning with the August 25, 2006 edition of SmackDown!, vignettes began airing featuring Akio dressed as a cowboy and calling himself Jimmy Wang Yang, in the process cutting his previously long hair short and growing a Fu Manchu mustache to fit the gimmick. In the vignettes, Yang questioned why people would assume stereotypical Asian things about him while proclaiming that he was much more of a redneck, of which he was proud. The vignettes played for several weeks, with Yang focusing more on his redneck leanings. On the September 29 SmackDown!, he made his in-ring debut in a loss to Sylvan, who illegally used the ropes for leverage to get a pin.
In December 2006, Yang became the number one contender for the Cruiserweight Championship. Yang challenged Gregory Helms for the Cruiserweight Title, but lost to him at Armageddon. Later, he participated in a Cruiserweight Open at No Way Out for the Cruiserweight Title, but was unable to win the title. In mid-2007, Yang and Chavo Guerrero began a feud. On the June 15 edition of SmackDown!, Yang once again became the number one contender for the Cruiserweight Championship, after feuding with Guerrero for almost two months. At Vengeance: Night of Champions, Yang lost to Guerrero after a frog splash. During The Great American Bash, Yang competed in another Cruiserweight Open and, although he was never pinned, Hornswoggle won the title after becoming an official entrant in the match due to him hiding under the ring. On the July 27 edition of SmackDown!, Yang teamed with Torrie Wilson against Kenny Dykstra and Victoria; Yang and Wilson won the match after Yang pinned Dykstra. Following this match, Wilson began appearing as Yang's new valet.
On October 12, on SmackDown!, Jamie Noble, in order to weed out the competition for the vacant Cruiserweight Championship, told Deuce 'n' Domino that Yang had been making "lewd remarks" about Domino's on-screen sister and manager, Cherry. Deuce demanded a match against Yang from acting General Manager Vickie Guerrero. Yang won the match with his signature moonsault from the top rope, but was attacked by Deuce, Domino, and Cherry after the match. The next week, Yang and Shannon Moore teamed up against Deuce 'n' Domino in a tag team match, which Yang and Moore won. On December 18, Yang and Moore defeated the WWE Tag Team Champions John Morrison and The Miz to earn a title-shot, igniting a feud between the two teams. On January 1, 2008, Yang and Moore once again beat Morrison and the Miz, this time in a six-person tag team match also involving Layla and Kelly Kelly. The following week, in a Fifteen Minutes of Fame Match, Moore and Yang tied the champions on pinfalls, resulting in Miz and Morrison retaining the titles.
On June 9, 2008, Yang was suspended for thirty days for his first violation of the WWE Wellness Policy. He made his return on the July 18 edition of SmackDown, being defeated by the repackaged The Brian Kendrick. In September 2009, Yang sustained cuts and bruises to his back from Kane after being attacked with a Singapore cane on an episode of WWE Superstars.
In late 2009, Yang formed a tag team with Slam Master J and in their first match as a team, they lost to The Hart Dynasty on the December 10 episode of Superstars. On the December 18 episode SmackDown, Yang and J lost a rematch to the Dynasty. On the January 14, 2010 episode of Superstars, Yang and J scored their first win as a team after they defeated Mike Knox and Charlie Haas. On April 22, 2010, Yang was released from his WWE contract.