Cullen has devoted his professional career to advocacy on behalf of poor and marginalized individuals and communities. After college, Cullen spent four years living in Anchorage, Alaska, working as an AmeriCorps VISTA volunteer with Native Alaskan communities and as Executive Director of Habitat for Humanity-Anchorage. In 1999, Cullen returned to school at the University of Wisconsin and obtained both a J.D. and a Master’s Degree in Public Policy.

After working for two years with a federal judge in Cleveland, Cullen joined the Cuyahoga County Public Defender’s Office in its appellate division in 2005. At the Public Defender’s Office, Cullen has handled hundreds of appeals in the Eighth District Court of Appeals, authored several amicus briefs in the Ohio Supreme Court, and has personally argued more than a dozen cases before the Ohio Supreme Court. He has also handled numerous cases in the Cuyahoga County Common Pleas Court and litigated habeas corpus petitions in federal court. In 2017, Cullen became both the supervisor of the Appellate Division and the Deputy Chief Public Defender. He served in that capacity until his appointment in 2021 as Chief Public Defender.