Resigning from the army in 1836, he became a civil engineer and constructor of railways, and was engaged under the war department in survey work.
Resigning the secretaryship in 1848, he was elected to the national House of Representatives as an anti-slavery Whig to succeed John Quincy Adams, and was re-elected in 1849, and, as an independent candidate, in 1850, serving until March 1853.
On the 10th of December 1701 Newton resigned his professorship, thereby at the same time resigning his fellowship at Trinity, which he had held with the Lucasian professorship since 1675 by virtue of the royal mandate.
Resigning his post on the 5th of February 1871, he was in January 1872 elected to the National Assembly, and in 1876 to the Senate.
Resigning his commission in 1857, McClellan became successively chief engineer and vice-president of the Illinois Central railroad (1857-1860), general superintendent of the Mississippi & Ohio railroad, and, a little later, president of the eastern branch of the same, with his residence in Cincinnati.