Marshall Hanley is originally from Muncie, Indiana where he lived until leaving for college. Marshall graduated from Northwestern University in Evanston, Illinois in 1974 and received his undergraduate degree in communication studies. Following receipt of his undergraduate degree, Marshall attended and graduated from the Indiana University School of Law in Indianapolis, cum laude, in 1977. Upon graduation from law school, Marshall served a two year clerkship for the Honorable S. Hugh Dillin of the United States District Court for the Southern District of Indiana. During his tenure as a clerk for Judge Dillin, the Indianapolis Public School Desegregation case was at the height of its controversy. After completing his clerkship in 1979, Marshall joined the law firm of Bingham Summers Welsh & Spillman (now Bingham McHale).

Following several years of practice at Bingham Summers, Marshall determined that his interest in civil litigation was in the representation of patients and other personal injury victims, so he left Bingham Summers and practiced for the next fifteen (15) years with the personal injury law firm of Mitchell Hurst Jacobs & Dick in Indianapolis. In 1999, Marshall joined the law firm of Findling Garau Germano & Pennington, P.C. as a partner. The firm is now known as Garau Germano Hanley & Pennington, P.C. and Marshall continues to concentrate his practice in the areas of medical malpractice, wrongful death, and serious personal injury.

In 2004, Marshall was selected as one of two Trial Lawyers of the Year by the Indiana Trial Lawyers Association. Marshall considers this recognition to be the pinnacle of his professional achievements. Marshall is also a member of the American Association for Justice, the American Society of Law & Medicine, and is a former member of the Indiana Commission on Higher Education, serving under then Governor Evan Bayh.