The earliest known true transsexual genital conversion surgery of any kind was performed in 1882 on a trans man named Herman Karl.[1] However, "earliest transsexual genital conversion surgery" depends on one's definition. Eunuchs have been around for all of human history, and while many eunuchs consider themselves cisgender men, many others consider themselves another gender that isn't female or male, such as hijra. Some sources credit the first trans male genital conversion surgery as, instead, the one performed on a trans man named Michael Dillon in the 1930s, perhaps depending on how one defines that surgery.
↑James Sears, Gay, Lesbian and Transgender Issues in Education. p. 109. Google Books link