In 1852, McCulloch moved to Peoria, Illinois and opened a classical school in the basement of First Methodist Church. In 1854, he read law at the partnership of Manning & Merriam. In fall 1855, he was elected as a school commissioner for Peoria County, Illinois, a post he would hold for six years.
McCulloch was admitted to the bar on September 2, 1858. In 1865, he was appointed to fill a vacancy as a prosecutor. Building on his prominence as a prosecutor, McCulloch ran successfully for circuit judge, and was re-elected in 1879. Shortly after he was re-elected, he was appointed to the Illinois Appellate Court, a post he held for six years. In 1880, he served as the second president of the Illinois State Bar Association. In 1883, he ran unsuccessfully for a seat on the Supreme Court of Illinois, losing to Alfred M. Craig.