Dawson died at his residence in the suburb of Bethesda, Md., said a grandson, Chris Coyner.
As administrative assistant to Truman from 1947 to 1953, Dawson was responsible for all presidential appointments. In 1948, he helped organize Truman's whistle-stop presidential campaign and served on Truman's election strategy board.
He also was chairman of the White House Loyalty Review Board, which was established by Truman to oversee federal loyalty investigations of government employees.
Dawson is survived by his wife, Virginia J. Dawson, two daughters, a son, four grandchildren and one great-grandchild.