William Patrick Clark was born on October 23, 1931. He attended Stanford University and then Loyola Law School. He also served in the United States Army Counter Intelligence Corps.

Clark was a judge for the Superior Court of California from 1969 to 1971 and an associate justice on the California State Supreme Court from 1973 to 1981. Clark served President Ronald Reagan in several capacities: as deputy secretary of state (1981-1982), as assistant to the President for national security affairs (1982-1983), and as secretary of the interior (1983-1985).Following his time in the cabinet, President Reagan appointed Clark chair of the Task Group on Nuclear Weapons Program Management. He also served as a presidential emissary to the chairmen of the Navajo and Hopi Indian tribes, as a member of the Commission on Defense Management (also known as the Packard Commission), and as a member of the Defense Department's Commission on Integrated Long-Term Strategy. Clark is also a trustee of the Ronald Reagan Presidential Foundation and Library.

Currently, Clark is the chief executive officer of Clark Company and is senior counsel to the law firm of Clark, Cali and Negranti.