Cho excelled in his tasks as a security guard because of his prior military experience. He served as a U.S. Marine for eight years, spending time in front line war zones such as Somalia, Africa. He carried a .45 handgun and was trained to use it.
He was a highly educated soldier serving as a medic first class while in active duty. Subsequently, he worked as a doctor in the civilian medical industry for a year but became disillusioned with the indusry. While Cho didn't offer a reason for the disillusionment, Sonny guessed correctly, that Cho was sued and then stop practicing shortly afterwards.