He was a member of the Australian Labor Party (ALP) and served as a Senator for Western Australia from 1950 to 1975.

[3][4] According to Kim Beazley he was a "... key assistant to Gough Whitlam as he set about the task of restructuring the Labor Party ... and made an intelligent, brilliant rabble fit for government.

Australia, if it is to serve national interests in an effective manner, can no longer afford to impose on international events interpretations at variance with the facts.

"[7] This approach translated into action in various ways, on 26 February 1973, Willesee led the push to recognise the Democratic Republic of Vietnam, and directed the establishment of the Australian Embassy in Hanoi in July 1973.

During the 1970s Willesee worked for the United States of America in what a historian has called "a discreet relationship".

At his death, the Prime Minister said: In my acquaintance with him—and I know I speak for those of my party and the National Party who dealt with him when he was a member of parliament—he was a friendly, decent, courteous and forthright man, whom we respected across the political divide.According to the leader of the opposition at the time, Don was a great human being, a man of immense integrity.