In 1922 he married Dorothy Brooks Johnston. Notwithstanding his prodigious financial accomplishments, Killam was a very reserved man who eschewed publicity and was virtually unknown outside a small circle of close acquaintances. Killam died in 1955 at his Quebec fishing lodge.
Having no children Killam and his wife devoted the greater part of their wealth to higher education in Canada. The Killam Trusts, established in the Will of Mrs. Killam, are held by five Canadian Universities. Dalhousie University, in Halifax, benefited the most and the library there is known as the Killam. Money also went to establish Izaak Killam Hospital for Children in Halifax and the Montreal Neurological Institute. The current market value of the Killam endowment is approximately $400 million Canadian dollars and it is used to fund research and artistic ventures across Canada.
When Killam died the government, at his request, used his inheritance taxes, coupled with those of Sir James H. Dunn and a large donation, to establish the Canada Council for the Arts.