Mary McDade was born in Columbia, South Carolina, in August 1939 and was raised in Ann Arbor, Michigan. She graduated with a B.A. in Sociology from the University of Michigan in 1961. In 1963, she married Joe Billy McDade and for 18 years worked primarily as the mother of their four children, Rebecca, William, Sarah, and Matthew. During that time, she was elected to the Peoria Board of Education, serving as president in 1972-73; was appointed to the Peoria Public Library Board, and was a member of the Board of Trustees of Eureka College, serving as chairman of the Board from 1980 to 1982. She worked actively in the areas of education, mental health, drug abuse, youth outreach, and civil rights.
In 1981, at the age of 41, she enrolled in the University of Illinois College of Law, earning her J.D. in 1984. She was a law clerk for the Honorable Michael Mihm, United States District Judge for the Central District of Illinois from 1984-86. She joined the law firm of Quinn, Johnston, Henderson & Pretorius in Peoria in 1986, becoming a partner in the firm in 1991. She has been an active participant in programs and projects of the American, Illinois, and Peoria County Bar Associations. She was named an Outstanding Alumnus of the University of Illinois College of Law in 2000. She is admitted to practice in the State of Illinois, United States District Courts for the Central and Northern Districts of Illinois, United States Court of Appeals for the Seventh Circuit, and the United States Supreme Court. Judge McDade is the first African-American female elected to the Appellate Court outside of Cook County.