The murder of Cho Joong-pil, 22, in the Itaewon nightlife district, which is popular with expatriates in the South Korean capital, fueled discontent towards U.S. military bases in the country.
South Korean officials said Arthur John Patterson, whose father was a civilian contractor to the U.S. military, fled to the United States after Cho was found dead on the floor of the restaurant bathroom with stab wounds to his neck and body.
Patterson, 36, was extradited to South Korea last year. He told the court he had been living in Los Angeles.
“The court decided on a life sentence, but in view of the defendant’s age, of being under 18 at the time, the sentence will be 20 years in prison,” Judge Shim Kyu-hong said.
The case has drawn intense public interest in South Korea, where the conduct of U.S. military personnel is scrutinized.
This month, Patterson repeated his claim of innocence in the closing arguments of his trial.
Patterson’s friend, Edward Lee, also a U.S. citizen, was initially sentenced to life in prison for the murder but was later acquitted on appeal due to lack of evidence.
Both men admitted to witnessing the murder but accused each other of killing Cho.
Patterson’s lawyer could not immediately be reached after the verdict for comment on whether he would appeal.
There are some 28,500 U.S. troops based in South Korea.