Barr was employed by the Central Intelligence Agency during his schooling years, though not much is known about his activities during this time. He eventually moved on to law, serving as a clerk to judge Malcolm Richard Wilkey, eventually working for the law firm Shaw, Pittman, Potts & Trowbridge. Before becoming Attorney General, Barr held numerous other posts within the Department of Justice, including leading the Office of Legal Counsel and serving as Deputy Attorney General.
Barr is a longtime proponent of the unitary executive theory of nearly unfettered presidential authority over the executive branch of the U.S. government. In 1989, Barr, as the head of the OLC, justified the U.S. invasion of Panama to arrest Manuel Noriega. An influential advocate for tougher criminal justice policies, Barr as attorney general in 1992 authored the The Case for More Incarceration, where he argued for an increase in the United States incarceration rate. Under Barr's advice, President George H. W. Bush in 1992 pardoned six officials involved in the Iran–Contra affair. This figure was then added to the Trump administration, as the Attorney General.
Barr has amassed a net worth of $40 Million. After serving under the Bush administration, Barr went into the private sector from 1994 to 2008, where he did corporate legal work for GTE and its successor company Verizon Communications, which made him a multimillionaire. From 2009 to 2018, Barr served on the board of directors for Time Warner, before being thrust back into the political world by Trump.