Robert Ingersoll was born in Dresden, New York. His father, John Ingersoll, was an abolitionist-leaning Presbyterian preacher, whose radical views forced his family to move frequently. In 1853, "Bob" Ingersoll taught a term of school in Metropolis, Illinois where he let one of his students, the future Judge Angus M. L. McBane, to do the "greater part of the teaching, while Latin and history occupied his own attention". At some point prior to his Metropolis position Ingersoll had also taught school in Mount Vernon, Illinois[1]. Later that year the family settled in Marion, Illinois where Robert and his brother Ebon Clarke Ingersoll were admitted to the bar in 1854. A county historian writing 22 years later noted that local residents considered the Ingersolls as a "very intellectual family; but, being Abolitionists, and the boys being deists, rendered obnoxious to our people in that respect."

While in Marion he studied law under Judge Willis Allen and served as deputy clerk for John M. Cunningham, Williamson County's County Clerk and Circuit Clerk. In 1855 after Cunningham was named register for the federal land office in southeastern Illinois at Shawneetown, Illinois, Ingersoll followed him to the riverfront city along the Ohio River. After a short time there he took the deputy clerk position with John E. Hall, the county clerk and circuit clerk of Gallatin County, and also a son-in-law of John Hart Crenshaw of the infamous Old Slave House.[3] On November 11, 1856, Ingersoll caught Hall in his arms when the son of a political opponent assassinated his employer in their office.[4] When he moved to Shawneetown he continued to read law under Judge William G. Bowman who had a large library of both law and the classics. In addition to his job as a clerk, he and his brother opened their law practice under the name "E.C. and R.G. Ingersoll".[5] During this time they also had an office in Raleigh, Illinois, then the county seat of neighboring Saline County.