Before coming to Rutgers, Cahill was a tenured faculty member at Brooklyn Law School, where he also served as associate dean for academic affairs (2010-13) and as vice dean (2013-15). During his time in Brooklyn Law's administration, Cahill was involved in nearly every aspect of the school's operation, including curricular oversight and reform, support for faculty research, admissions and financial aid policy, budgetary planning, supervision of administrators and staff, maintenance and improvement of school facilities, external relations, and fundraising.

Cahill received a B.A. from Yale University and J.D. and M.P.P. degrees from the University of Michigan. After graduating from law school, where he was a note editor for the Michigan Law Review, he served as a law clerk to Judge James B. Loken of the U.S. Court of Appeals for the Eighth Circuit. He was then involved as staff director and consultant, respectively, for major criminal code reform projects in the states of Illinois and Kentucky.

Cahill's scholarship focuses primarily on criminal law, though he has also written and taught courses about health law and policy. His criminal law work includes three books written with Paul H. Robinson: the general one-volume treatise Criminal Law (Aspen, 2d ed., 2012), the student casebook Criminal Law: Case Studies and Controversies (also with Shima Baradaran Baughman; Wolters Kluwer, 4th ed., 2016), and Law Without Justice (Oxford University Press, 2006). Cahill's work has also appeared in the Northwestern University Law Review, Texas Law Review, Iowa Law Review, Washington University Law Review, and American Journal of Law and Medicine, among other publications.

Before joining the Brooklyn Law School faculty in 2003, Professor Cahill taught at Chicago-Kent College of Law as a visiting assistant professor of law.