In the Mexican War he had been aide-de-camp to General Scott.

He had been out of the army for some years before the rebellion, and was acting as professor of mathematics in St.
Xavier's College, Cincinnati, when he was appointed to the colonelcy of the Twenty-third Ohio upon Rosecrans's promotion.

Like Rosecrans, he was a Roman Catholic, though himself of Puritan descent.

It seems that at the time of the Puseyite movement in England and in this country there had been a good many conversions to Romanism among the students and teachers at West Point, under the influence of the chaplain of the post, and Scammon, among a number of young men who subsequently became distinguished officers, was in this number.

It need hardly be said that Scammon was well instructed in his profession.