Hayes moved to Cincinnati in 1849, and established a law practice in that growing city. He handled many different types of cases, including criminal defense. He pleaded an insanity defense on behalf of a disabled girl named Nancy Farrer. Though she was convicted and sentenced to death, the prosecutors and others praised Hayes for his able defense and deftness in courtroom argument. Hayes pursued an appeal on behalf of Farrer, and she eventually was placed in an insane asylum. In December 1853, he formed another law firm called Corwine, Hayes & Rogers.