The 1864 Democratic National Convention nominated a ticket of George B. McClellan, who favored continuing the war, and Pendleton, who opposed it.

Pendleton was a strong contender for the presidential nomination at the 1868 Democratic National Convention, but was defeated by Horatio Seymour.

After the assassination of President James A. Garfield, he wrote and helped pass the Pendleton Civil Service Reform Act of 1883.

[2] A leading defender of slavery,[5] he was a leader of the "peace" faction of his party during the American Civil War, with close ties to the Copperheads.

George E. Pugh, the Democrat nominated to run for Pendleton's seat, lost to Republican Benjamin Eggleston.

Following the 1881 assassination of James A. Garfield, he passed his most notable legislation, known as the Pendleton Act of 1883, requiring civil service exams for government positions.

[4] Instead, President Grover Cleveland appointed him Envoy Extraordinary and Minister Plenipotentiary to Germany the year that he left office, which he served until April 1889.

[4] Pendleton had a very Jacksonian commitment to the Democratic Party as the best, perhaps the only, mechanism through which ordinary Americans could shape government policies.

Mach (2007) argues that Pendleton's chief contribution was to demonstrate the Whig Party's willingness to use its power in government to achieve Jacksonian ideals.

While his Jacksonian commitment to states' rights and limited government made him a dissenter during the Civil War, what Mach calls Pendleton's Jacksonian "ardor to expand opportunities for ordinary Americans" was the basis for his leadership in civil service reform and his controversial plan to use greenbacks to repay the federal debt.

What appeared to be a substantive ideological shift, Mach argues, represented Pendleton's pragmatic willingness to use new means to achieve old ends.

[13] In Steven Spielberg's 2012 film, Lincoln, Pendleton is played by Peter McRobbie and portrayed as one of the most notable opponents of the Thirteenth Amendment and of the Civil Rights Act of 1866.