[1] In May 1843, at the age of 18, he left his parents' home in Cayuga[clarification needed] and moved to Auburn, where he entered the firm of Beach & Underwood as a law student.

At the end of his second term he was chosen to be a Member of the New York Assembly by the Republicans to represent the second district of Cayuga and served in the legislature in 1857 but declined renomination.

On July 4, 1861,[4] he took his seat at the extra session of the 37th Congress convened by President Abraham Lincoln, right after the start of the Civil War.

[5] He was referred to as the youngest-looking member on the floor by Washington newspaper correspondents, who described him as follows: Mr. Pomeroy of Auburn is small in stature, with keen black eyes, a peculiarly expressive countenance and somewhere near as smart as chain lightning, at least when he deals with lower law Democracy.

He is one of the most energetic and effective debaters in the House and brimful to running over with that kind of Republicanism which is found in the now somewhat antiquated document known as the Declaration of Independence.

The lions of buccaneer Democracy fare hard when they fall into his hands and he occasionally handles certain old fogy Republicans without gloves.

Upon his resignation, the House passed a motion declaring Pomeroy, who was himself leaving Congress the next day, duly elected as speaker in place of Colfax.

In the spring of 1866, the Merchants Union Express Company was organized to transport trade and goods across the United States with Elmore P. Ross as president, William H. Seward Jr. as vice-president, John N. Knapp as secretary, William C. Beardsley as treasurer, and Pomeroy as their attorney.