Cohn was born Itsak Leib Jaszovics in Berezna, Hungary and orphaned when he was seven. I have been unable to discover who cared for him after that.
The story he told of himself was that he became bar mitzvah at 13, studied for the rabbinate and was ordained.
He married and had children in Hungary, before emigrating to the United States alone in 1892, after coming into conflict with the Hungarian Jewish community over the interpretation of passages relating to the coming of the Messiah in the Talmud and the Book of Daniel. Soon after arriving he met a group of Messianic Jews: Jews who believe that Jesus was the Messiah. but retain their Jewish identity and religious practices. He was soon converted and became an evangelist himself, preaching to other Jews. He studied at a Christian seminary in Scotland in 1893 and his family, who also converted, joined him in America.
Cohn founded the American Board of Missions to the Jews (now called Chosen People Ministries), the first organized Jewish-led American evangelical mission to the Jews. When he died he was succeeded by his son, Joseph.
Cohn is reviled as an apostate by most mainstream Jews, who maintain that a main pillar of the "proof" that Jesus is the Messiah is a forgery inserted into the Zohar, possibly by Cohn himself.
It is hard to know whether his own autobiography was sanitized, or how much credence to give to the stories that he was a saloon-keeper and had been convicted of forgery in Hungary.