In the 1900s Killen was secretary of a branch of the Peace and Humanity Society, which opposed Australia's involvement in the Boer War, when British troops were "depriving a brave and free people ... of their liberty and independence", indulging in "farm burning, dam-cutting, and general destruction", and "barbarous treatment of women and children ... in direct violation of the rules of warfare, as laid down by the Hague Conference".

He was also president of the Murrumbidgee Shire, a Yanko Shire councillor, vice-president of the Riverina New State League, a member of the councils of the Graziers' Association and Stockowners' Association and a member of the Australian Meat Council, as well as his local pastures protection board.

[9] In 1922 he was elected to the Australian House of Representatives as a member of the Country Party, defeating Nationalist MP John Chanter for the seat of Riverina.

Killen held Riverina until his retirement in 1931, having had a heart attack in 1929 and struggled with health issues during his final term.

[10][11][12][13] Killen returned to farming after his retirement from politics and also continued a long-running role as a director of the Farmers' and Graziers' Co-operative Grain Insurance and Agency Co. Ltd.