In his Springfield practice, Lincoln handled "every kind of business that could come before a prairie lawyer". Twice a year he appeared for 10 consecutive weeks in county seats in the Midstate county courts; this continued for 16 years. Lincoln handled transportation cases in the midst of the nation's western expansion, particularly river barge conflicts under the many new railroad bridges. As a riverboat man, Lincoln initially favored those interests, but ultimately represented whoever hired him. He later represented a bridge company against a riverboat company in Hurd v. Rock Island Bridge Company, a landmark case involving a canal boat that sank after hitting a bridge.

Lincoln appeared before the Illinois Supreme Court in 175 cases; he was sole counsel in 51 cases, of which 31 were decided in his favor. From 1853 to 1860, one of his largest clients was the Illinois Central Railroad. His legal reputation gave rise to the nickname "Honest Abe".

Lincoln argued in an 1858 criminal trial, defending William "Duff" Armstrong, who was on trial for the murder of James Preston Metzker. The case is famous for Lincoln's use of a fact established by judicial notice to challenge the credibility of an eyewitness. After an opposing witness testified to seeing the crime in the moonlight, Lincoln produced a Farmers' Almanac showing the moon was at a low angle, drastically reducing visibility. Armstrong was acquitted.

Leading up to his presidential campaign, Lincoln elevated his profile in an 1859 murder case, with his defense of Simeon Quinn "Peachy" Harrison who was a third cousin;Harrison was also the grandson of Lincoln's political opponent, Rev. Peter Cartwright. Harrison was charged with the murder of Greek Crafton who, as he lay dying of his wounds, confessed to Cartwright that he had provoked Harrison. Lincoln angrily protested the judge's initial decision to exclude Cartwright's testimony about the confession as inadmissible hearsay. Lincoln argued that the testimony involved a dying declaration and was not subject to the hearsay rule. Instead of holding Lincoln in contempt of court as expected, the judge, a Democrat, reversed his ruling and admitted the testimony into evidence, resulting in Harrison's acquittal.