Allen had been set to go on trial for the bizarre incident later this week but reached a plea deal with prosecutors on 31 August and will serve 12 years behind bars.
He was arrested by police when the 28-year-old volunteer, who had flown to Oklahoma from Virginia, went to hospital because he was bleeding from the amateur surgery.
Allen pleaded no contest to felony counts of conspiracy to commit unlicensed surgery, practising medicine without a licence and attempted distribution of a psychedelic mushroom.
He also pleaded no contest to two misdemeanour counts and was sentenced to two years in jail and 10 years in prison.
Allen told the judge he believed the plea was in his best interest because “a jury might convict me,” according to The Oklahoman.
His husband, Thomas Evans Gates III, 42, was also charged in this case and pleaded guilty to three misdemeanour charges last month.
Investigators were told by the victim that Allen claimed he had been performing gender reassignment surgeries for 15 years and had six clients ready to go under the knife.
The victim made contact with Allen through his website Eunuch Maker while searching online about castrations.
He flew to Dallas, Texas, and the couple drove the volunteer to their remote cabin in the southeast part of the state, where Allen performed a two-hour surgery using only local anaesthetic.
“After the surgery was over Allen said that he was going to consume the parts and laughed and said that he was a cannibal,” the victim told investigators, an affidavit in the case stated.
Allen and Gates were arrested after they visited the victim in hospital.
Allen has denied any claims of cannibalism.