In 1968, Williams started work as a barrister. In 1971, he became counsel for the Asian Development Bank. However, four years later, he returned to practicing law on his own. He was appointed Queen's Counsel in 1982, and became a Member of the Order of Australia in 1989. Williams continued to practice law until his election to Parliament in 1993.

Williams was briefly a member of the Opposition Shadow Ministry in 1994, serving as Shadow Attorney-General and Shadow Minister Assisting the Leader of the Opposition on Constitutional Reform.

In 1996, when the Liberals won office, Williams was appointed to the Cabinet as Attorney-General. He served in this capacity until 2003. Williams was also Minister for Justice for a period in 1996-97. He had also attended the 1998 Constitutional Convention as a parliamentary delegate.

After the Liberal ministerial shakeup of 2003, Williams was appointed Minister for Communications, Information Technology and the Arts. In April 2004, Williams announced he would not be contesting the 2004 election. He stood down from the ministry in July 2004.