[2] Sullivan began his legal career in New Orleans, Louisiana as a law clerk for Judge John Minor Wisdom of the United States Court of Appeals for the Fifth Circuit.
In 1976 he joined the law firm Jenner & Block in their Chicago office and was promoted to partner in 1981.
[3][4] In 1994 Sullivan accepted the position of dean of the Washington and Lee University School of Law.
He is the holder of the Cooney & Conway Chair in Advocacy at Loyola University Chicago School of Law.
Sullivan was also a member of the Legal Review and Advisory Panel of the National Leadership Coalition on AIDS from 1992 to 1998.