Shuo-Shan Wang, 29, told authorities he had taken the knife to more than 50 men, a U.S. court heard.
Wang, in the U.S. on a student visa, advertised his unusual services on a Web site of an international network of men whose fetish was to have their testicles removed, the court heard.
A jury found Wang guilty of practicing medicine without a license and dispensing prescription medicine without a license. He faces up to four years in jail and will be sentenced later this month.
Wang earlier withdrew a guilty plea after a Michigan judge rejected a recommendation for probation. The judge said he would sentence Wang to at least six months jail.
Wang was arrested 10 months ago after police found a 48-year old man hemorrhaging outside his home in Detroit, Michigan, following a voluntary castration.
The man sought the castration because he had a sexually transmitted disease and no longer wanted a sex drive, prosecutors said.
Authorities said Wang performed the castration on his kitchen table and then he and the man shared dessert at the same table. The man was not charged for the 40-minute surgery.
Wang's neighbors called the police after they saw the bloodied man wandering along the suburban street. Police later found the man's testicles in a jar in Wang's fridge.
Detroit prosecutor Ken Frazee told the court Wang initially admitted to authorities he learnt the surgery from his grandfather, a doctor in Taiwan, the Sunday Mail reported.
Wang told authorities he performed his first surgery on a dog, and then on the dog's owner and three of the owner's friends in Australia several years ago, Frazee said.
"Wang advertised on a fetish Web site for the procedure which is carried out while the patient is conscious," the Mail quoted Frazee as saying.
"He was flown all around the U.S. and world by patients ... After 10 years in this job and seeing some extreme things, this is the most unusual case I have ever had to deal with."