Stidham and Sanchez were convicted of a third-degree felony, which is punishable by up to five years in prison. They will request probation only.
After leaving the courthouse, Stidham, who used to be a sheriff's deputy in Salt Lake and Tooele counties, said he was "shocked" by the verdict.
"If you had watched the testimony, there was no way in hell I was going to be convicted of anything," he said.
Another co-defendant, Aaron Sawyer, 31, pleaded guilty before the trial to misdemeanor assault and also will seek probation. Both Sawyer and Sanchez are mixed martial-arts (MMA) fighters.
The convictions have implications for MMA in Utah. Stidham, 43, runs the state's biggest promotion, the Ultimate Combat Experience, which hosts sanctioned fights around the state. The director of the state's athletic commission had said it would review Stidham's license if he were convicted.
The defendants were celebrating new recruits into their league when they went to Southern X-posure in South Salt Lake on Jan. 11, 2009. The bouncer involved in the fight, Jacob Alba, testified a dancer told him the defendants were making crude comments. Alba said he confronted the men. There was conflicting testimony about who challenged whom to a fight.
Alba testified as he and the defendants were walking toward the door, Sanchez lurched backward into Alba. When the men moved into view of a surveillance camera, Alba is seen pushing Stidham, then punching at Sanchez and then back at Stidham. A melee ensued with Alba and Stidham on the floor and others running to both men.
Alba testified the beating fractured an eye socket and broke his nose so severely he needed nasal surgery. Alba also said he still suffers migraines.
Alba did not return calls for comment Wednesday.
Deputy Salt Lake County District Attorney George Vo-Duc acknowledged Alba initiated the fight, but Vo-Duc also described Stidham sitting on top of Alba and hitting Alba's head while Sanchez and Sawyer kicked Alba.
"When you're going to the 40th second of the beating, the 50th second of the beating, the right to self-defense has evaporated, especially when it's three on one," Vo-Duc said in his closing argument.
The video, which was played in the courtroom, also showed someone swinging what Stidham's attorney claimed was a knife that cut the top of Stidham's head. Neither the knife-wielder nor any club staff were charged. Vo-Duc argued the defense was pointing to the knife to distract attention from Stidham and Sanchez. Stidham's attorney, Tyler Ayres, said the knife escalated the fight.
Stidham, who took the witness stand on Wednesday, said he was on top of Alba when he suddenly felt blood in his eyes and mouth. Stidham called it a sensation he had not felt in all his years of fighting and it caused him to increase his beating on Alba. Ayres contended Alba picked a fight with the defendants. Ayres played a portion of the surveillance video from before the fight where a club employee issues Alba a "double-dog dare" to "mess up" the defendants. Ayres and defense attorney Scott Berrett, who represented Sanchez, also argued state law does not define when an attacker is neutralized and force is no longer necessary.
Stidham said even if he loses his promoters license, he might be able to stay with his company as a consultant and let someone else be the license holder.
"I keep thinking what I could have done differently and I can't really think of anything," Stidham said. "But I shouldn't have been there [at Southern X-posure] in the first place."