McKenna was the man who unearthed the racist rant tapes of Los Angeles cop Mark Fuhrman that undermined the prosecution of Simpson. He also helped Kennedy scion William Kennedy Smith beat rape charges in another high profile case.
But the Casey Anthony trial is special for McKenna.
"This case was the sweetest victory of all the cases I have ever worked because there was so much hatred toward Casey and her lawyers. We had no money, no resources and we were working against a firestorm of bad press," McKenna said.
Anthony's lawyer, Jose Baez, told ABC News that she is facing a tougher "uphill battle" than OJ. Simpson had.
"Casey's current fight is a lonelier one too," he said.
Although Simpson spent several years after his acquittal largely golfing, in 2008 he was convicted of robbery and sentenced to 33 years in prison.
Baez says Anthony has been passing the time since her release from jail reading and is committed to getting well. He says she is still grieving from the more than three years of traumatic events since her daughter Caylee died.
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She was acquitted of charges she murdered Caylee, but spent three years in jail waiting for a trial that tore apart her family. Since her acquittal Casey Anthony has received death threats and been ordered to return to Florida to serve a year's probation.
"Casey is at a much bigger disadvantage than OJ because he had money, a support system and he was a significant part of the African-American population who believed he was innocent. There was no division beyond racial lines with Casey," Baez said.
McKenna reflected on both verdicts saying, "I was more surprised by Casey's acquittal than OJ's because I had more confidence in OJ's jury. There was just more science to back up his case than Casey's."
McKenna, who is still in touch with Simpson, said the former football great and star of the "Naked Gun" movies, was an excellent witness for himself, which was another bonus for him. Casey Anthony didn't testify at her trial.
While Simpson had a fan following, Anthony was an unknown single mother reviled for not telling police for a month that her child was missing.
"The media attention and absolute conviction by the public that Casey and OJ were guilty early on, without any consideration of the evidence, was so intense in both cases," McKenna said.
"But they had stark contrasts too. Casey had absolutely no money, and OJ had millions of dollars. Casey's lawyer, Jose Baez, was a young, fairly new lawyer compared to OJ's high profile lawyer, Johnnie Cochran. Baez was an underdog," he said.
For the first year of Casey Anthony's case, McKenna admits he did not follow the press reports and knew few details. He got a phone call from Casey's defense team the day Caylee Anthony's remains were found.
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Still knowing little about the case, McKenna's first move was a visit to the Anthony house where he met Casey's parents, George and Cindy Anthony, and spent time in the bedroom Casey and Caylee shared.
"The room was a shrine of pictures to Caylee, hundreds of loving photographs. I was stunned by the neatness and organization of the room and all the photos. It was this lovely room, like a young kid that has a baby she loves. It was the opposite of what was being portrayed in the media," he said.
His first impressions of George and Cindy Anthony were not as favorable.
"I walked out of that house and thought to myself, 'I don't think this kid committed first degree murder. There is something really disturbing about this family. I was shocked by their lack of emotion. There was no sense of loss or despondency. Their granddaughter's remains had just been found and they're sitting around booking television interviews and talking about how much money their foundation is making," McKenna recalls.
He also says George and Cindy Anthony were not cooperative with him at first, even though he was there to help their daughter.
"George was bizarre. He would go around and around for hours and he wouldn't answer a question directly. And he is a former detective, so he knows the drill. There was something off with this guy," McKenna said.
He shared his instincts with Baez and suggested looking into the family dynamics - and any dark secrets -- of the Anthony family.
Some of the alleged family secrets famously unfolded at trial, with Baez accusing George Anthony of molesting Casey Anthony and having an affair during the search for Caylee.
"After I read the discovery and George's interviews, I thought, "This guy is way too eager to throw his daughter under the bus."
McKenna's theory of what happened to Caylee Anthony mirrors Baez's court defense of Casey Anthony.
"It was an accident that spun out of control and George disposed of the body, but I can't say exactly how," McKenna said. "If Casey wanted to be free, she would have dropped Caylee in the pool, called 911 and been a grieving mother."