His father, John Ingersoll was an abolitionist preacher. Ingersoll was born in Dresden, New York, but his family moved frequently because of his father's radical views before finally settling in in Peoria, Illinois. Ingersoll apprenticed himself to lawyers there and hung out his shingle.
With the advent of the American Civil War, he raised the 11th Illinois Cavalry Regiment and took command. The regiment fought in the Battle of Shiloh. Ingersoll was later captured, then paroled on his promise that he would not fight again. (This was common practice early in the war.)
After the war, he served as Attorney General of Illinois. He was a prominent member of the Republican Party, at that time the more progressive party. Although he never held any office, he was an active participant. His nominating speech for James G. Blaine in 1876 did not result in Blaine's candidacy, but the speech itself, known as the "Plumed Knight" speech, was considered the gold standard for political oratory.
Ingersoll was involved in several prominent trials as an attorney, notably, the Star Route trials, a major political scandal in which his clients were acquitted. He also defended a New Jersey man for blasphemy. Although he did not win acquittal, his vigorous defense is considered to have discredited blasphmeny laws and few other prosecutions followed.
Ingersoll was most noted as an orator, the most popular of the age, when oratory was public entertainment. He spoke on every subject, from Shakespeare to Reconstruction, but his most popular subjects were atheism and the sanctity and refuge of the family. He committed his speeches to memory although they were sometimes more than three hours long. His audiences were said never to be restless.