A year later, he moved to Marshall, Illinois to edit the Telegraph, a paper that supported the Know Nothing movement.
Callahan was elected as a justice of the peace in 1857, and it was at this time that he began to read law.
Callahan played a role in organizing the Illinois State Bar Association in 1877, and served as its president in 1889.
In 1883, Callahan gave a paper at the Illinois State Bar Association entitled "The Lawyers of the Bible."
The work was widely copied, and in January 1911, he was invited to deliver the paper as an address at the Indiana University School of Law.