After returning to the United States, LeBell competed as a heavyweight. In 1954 and 1955, while only 22 years of age, he captured both the heavyweight and overall Amateur Athletic Union National Judo Championships. His very first match was against John Osako, one of the highest ranked judokas at the national level. LeBell earned the upset win via a osaekomi. Afterwards, attracted by better potential earnings and the family's legacy in the Business, LeBell transitioned to professional wrestling. Despite his pedigree, he did not get over immediately with audiences but gradually became known for his martial arts background. He eventually adopted the role as "policeman" for the promotion, maintaining law and order, especially during matches involving his brother Mike LeBell. Gene also wrestled under a black mask as The Hangman, teaming up with Roy Staggs.

The match took place on December 2, 1963. The combatants were initially cautious, with LeBell being the first in pressing the action by attempting to throw Savage down. The boxer blocked the move, which aggravated an old shoulder injury of LeBell. LeBell tried several techniques through the second and third rounds and was finally successful in taking Savage down, but Savage kept defending both standing and on the ground in a very technical manner, seemingly confirming LeBell's theory about his opponent's judo training. Savage even attempted to sweep the judoka in one instance. Nevertheless, LeBell got mount and found the opportunity to execute an armbar, but he opted instead to seek a choke, concluding that Savage wouldn't surrender to a broken arm. Finally, he performed a left harai goshi in the fourth round and followed by locking a rear naked choke. Within seconds, Savage fell unconscious and LeBell was declared the winner.

Following his combat Sports career, LeBell, along with his brother Mike, ran the National Wrestling Alliance's Los Angeles territory, NWA Hollywood Wrestling from 1968 to 1982. In June 1976, LeBell refereed the infamous boxing-versus-wrestling contest between Muhammad Ali and Antonio Inoki in Tokyo, Japan. LeBell was selected from over 200 other applicants to Referee the bout. LeBell has opened two martial art schools in cooperation with others and has touted his 1963 match with Milo Savage as the first televised MMA fight in America.

LeBell also appeared in "The World of Martial Arts - Budojujitsu". This 1982 homevideo production, introduced by Chuck Norris and narrated by John Saxon, featured LeBell as a car thief and mugger who (in the prologue) accosts Mitsuru Yamashita and Budojujitsu creator Al Thomas...much to LeBell's regret. The program featured several other martial artists including Graciela Casilas, Karen Sheperd, Victor Ledbetter, Steve Sanders, brothers Benny and Reuben Urquidez, and Dave Vaden.

On March 23, 1991, LeBell was awarded the Honorary "Reel" Membership by the Ring of Friendship of the Cauliflower Alley Club. This award is only given to a select few. Others awarded have been James Cagney, Kirk Douglas, Karl Malden, Cesar Romero, Mickey Rooney, and Sylvester Stallone (movie stars that also did boxing and wrestling).

Along with the awards received for feats in judo and grappling, LeBell was the 2005 recipient of the Frank Gotch Award in celebration of the positive recognition he brought to the sport of wrestling, the Cauliflower Alley Club presented the award. On March 18, 1995, the Cauliflower Alley Club's again honored LeBell by presenting him with the "Iron Mike Mazurki" award; presented by one of his teachers, legendary professional wrestling champion Lou Thesz.

In 2000, the United States Ju-Jitsu Federation (USJJF) promoted him to 9th Dan in jujitsu and taihojutsu. On August 7, 2004, the World Martial Arts Masters Association promoted LeBell to 10th Degree and in February 2005, he was promoted to 9th Dan in Traditional Judo by the USJJF.

While serving as Stunt coordinator for the film Out for Justice, starring Steven Seagal, Seagal claimed that due to his aikido training he was 'immune' to being choked unconscious. It has been alleged that at some point LeBell heard about the claim, and gave Seagal the opportunity to prove it. LeBell is said to have placed his arms around Seagal's neck, and once Seagal said "go", proceeded to choke him unconscious. LeBell was requested to confirm the story publicly in an interview with Ariel Helwani in 2012, but he avoided answering the question, albeit implying that the incident was true. He was quoted as "When we had a little altercation or difference of opinion, there were thirty stuntmen and cameramen that were watching. Sometimes Steven has a tendency to cheese off the wrong people, and you can get hurt doing that." On the other hand, when Seagal was asked about the incident, he directly denied the allegations, calling LeBell a "pathological scumbag liar," and offered the name of a witness who could prove Lebell had fabricated this rumor. The claim garnered a heated response from LeBell's trainee Ronda Rousey, who assured Seagal was the one lying and declared "If [Seagal] says anything bad about Gene to my face, I'd have to make him crap his pants a second time."