In 1977, Manning began work as a corporate litigation attorney for Chicago-based United Airlines. A year later, Manning transitioned to the role of assistant United States attorney for the Northern District of Illinois, a role she held from 1978 to 1979. During that same time, Manning was an adjunct professor at the National Conference of Black Lawyers Community College of Law. In 1979, she started her judicial career as an associate circuit court judge in Cook County, where she served until 1986.[2]
Manning was a lead circuit judge in the Illinois Cook Judicial Circuit Court from 1986 to 1987.[citation needed] In 1987 she was elected as a justice in the Illinois First District Appellate Court of the Illinois Appellate Court, becoming the first African-American female member of the court.[2] From 1992 to 1994, Manning also worked as an adjunct professor at the DePaul University College of Law.[3]
Manning is known for sentencing Mark Whitacre, a whistleblower in the Archer Daniels Midlandlysine pricefixing case. Mark Whitacre's sentence was harsher than that of his superiors at the ADM company and has been often spotlighted for deterring future whistleblowers.[5]
