Williamson served more than a decade ago as state Republican chairman, from 1999-2001. In 1992, he was the Republican candidate who was defeated by Democrat Carol Moseley Braun.
Williamson died Sunday. He was 64 years old.
Williamson served three presidents in senior Foreign Service roles.
Illinois Republican Party Chairman Jack Dorgan urged Illinoisans to keep Williamson's wife and children in their thoughts and prayers.
"Rich was a true friend, and a great diplomat," Dorgan said. "He loved his family, loved his country, and loved this party."
"I am deeply saddened to learn of the passing of Amb. Rich Williamson, a dear friend and long-time foreign policy adviser," Sen. Mark Kirk said. "I extend my heartfelt condolences to Jane and the entire Williamson family. From the White House to the State Department to the United Nations to the International Republican Institute, Rich dedicated his life to spreading American values of freedom and democracy around the world. Like President Reagan, Rich believed in peace through strength, in that shining city upon a hill. I will miss his friendship and counsel."