She is a former writer and editor for National Geographic and the author of four books: The River of Doubt, a history of the Roosevelt–Rondon Scientific Expedition of the Amazon rainforest in 1913–14; Destiny of the Republic, about the assassination of James A. Garfield; Hero of the Empire, about Winston Churchill's activities during the Boer War; and River of the Gods, about the search for the source of the Nile River.

[3] She has a corner office at his publishing company in Overland Park, Kansas, where she works while their three children are in school.

In April 2012, Millard won the Edgar Award for Best Fact Crime Book for Destiny of the Republic.

[5] The book also received a PEN Center USA Award and the 34th Thorpe Menn Award for Literary Excellence from the American Association of University Women, Kansas City Branch, in 2012.

[6] Subsequently, Millard won the 2017 BIO Award from the Biographers International Organization for her biographies of Churchill, Garfield, and Roosevelt.