The war years for Montgomery were marked with personal loss. Just eight days after the announcement of war, on 13 August 1914, Maud's second son was born dead. The three Leaskdale men to whom she dedicated Rainbow Valley were ones she and Ewan knew through their church work. In 1916 she had published a book of poems, The Watchman and Other Poems, and had dedicated it "To the memory of the gallant Canadian soldiers who have laid down their lives for country and Empire." In May of 1916 she watched the Ontario County 116th Battalion march by her front door, wondering how many of them would ever come home again. Her half-brother, Carl, lost his leg at Vimy Ridge.

The war brought an even deadlier foe in its last months. A violent influenza pandemic, misnamed the Spanish Flu, claimed more lives than had the entire war effort. Montgomery had this flu, but recovered though her nerves were shattered for months afterwards. She watched helplessly as her dearest friend and cousin, Frederica Campbell Macfarlane, died of the flu on 25 January 1919. The war had begun and ended with personal tragedy, and Montgomery was a faithful recorder throughout of the sufferings of others and of the temper and heroism of the times.