In 1984 Williams received a master's degree from Johns Hopkins School of Advanced International Studies in Washington, D.C. She was a co-coordinator of the Nicaragua-Honduras Education Project (1984–86) and deputy director of Medical Aid for El Salvador (1986–92). In October 1992, with the cooperation of six international organizations, she coordinated the launch of the ICBL with the mission of abolishing the use of antipersonnel land mines (land mine). Her efforts bore fruit in December 1997, when the Mine Ban Treaty was signed by more than 100 countries in Ottawa. Within the following decade about 130 countries ratified the treaty—but not the major mine-producing ones, such as the United States, Russia, and China.

Williams lectured widely on the dangers of land mines, publicizing the presence of tens of millions of unexploded land mines in more than 70 countries. She was coauthor, with Shawn Roberts, of After the Guns Fall Silent: The Enduring Legacy of Landmines (1995), which examined the socioeconomic impact of land mine contamination in four countries. In 2007 she was appointed to lead a United Nations High-Level Mission to investigate human-rights abuses in the war-torn Sudanese region of Darfur.