Logan County was formed February 15, 1839, from Sangamon County by an act of the Illinois General Assembly. In 1841 Logan gained more land from Tazewell County. In 1847 a small part of DeWitt County became part of Logan County, completing the county boundaries as we know them today.

The county was named by Abraham Lincoln, a member of the General Assembly at the time and the prime mover in forming the county, in honor of his friend Dr. John Logan, a well known Illinois pioneer and fellow member of the Illinois General Assembly. Dr. Logan, father of Civil War General John A. Logan, was a resident of Murphysboro. He was not from Logan County.

Logan County covers approximately 618 square miles of central Illinois. Lincoln is the third and current county seat. Earlier county seats were Postville, later known as Camden, 1839-47, and Mt. Pulaski, 1847-53. The actual Postville courthouse is at the Henry Ford Museum in Michigan but a reproduction exists in Lincoln. The Mt. Pulaski courthouse, where Lincoln practiced law on the Eighth Judicial Circuit, still stands on the square in that town. It is one of only two still existing court houses where Lincoln actually practiced law on the circuit.

Shortly after the records were moved from Mt. Pulaski, the first courthouse in Lincoln burnt [April 15, 1857] destroying all the records that were housed there. Generally, records from the period before the fire Logan became a county in 1839 may be found at the State Archives or at IRAD in Springfield under Sangamon County. The records for 1839-1857 stored in court house were lost. Original federal land purchase records were not in the court house and still exist. Some deeds and other documents were brought in by the public after the fire and recreated. Some records for that period may show up in abstracts or legal files. The Logan County Genealogical & Historical Society has reconstructed some probate records.