While in Marion, he learned law from 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 registrar for the federal land office in southeastern Illinois at Shawneetown, Illinois, Ingersoll followed him to the riverfront city along the Ohio River. After a brief time there, he accepted 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. On November 11, 1856, Ingersoll caught Hall in his arms when the son of a political opponent assassinated his employer in their office.