A Democrat, Marshall worked in private legal practice in
Chicago from 1953 until 1967 at Johnston, Thompson, Raymond & Mayer (which later became known as
Jenner & Block), becoming partner in 1961. While at Jenner, Marshall also served as a special assistant attorney general for the state of Illinois from 1964 until 1967. Marshall also built a reputation for starting Jenner's pro bono legal program. Marshall then served as a law professor at the
University of Illinois College of Law from 1967 until 1973 and as a hearing officer for the Illinois Fair Employment Practices Commission from 1967 until 1972. In 1959, Marshall, then a resident of
Wheaton, Illinois, ran unsuccessfully as a Democrat for state's attorney in
DuPage County, a Republican stronghold. == Federal judicial service ==