From 1973 to 1977, Manning worked as a supervisory trial attorney for the Equal Employment Opportunity Commission in Chicago.
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][4] Manning is known for sentencing Mark Whitacre, a whistleblower in the Archer Daniels Midland lysine 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.