Key went to Cincinnati after graduation and began practice of the law, as partner of Alphonso Taft and William M. Dickson in April 1854.
He resumed practice of law in 1853, and continued in it, in partnership and alone, until the outbreak of the American Civil War.
Judge Key took an active and controlling part in securing unanimous action in the Ohio Legislature for the support of the National Government.
In this he was brought into contact with General McClellan, becoming Judge Advocate on his staff with the rank of Colonel, and received in an unusual degree the confidence of his commander.
While stationed in Washington, he drafted, and promoted the passage of, the bill abolishing slavery in the District of Columbia.