Armstrong held a B.A. (1910) from Sydney University with first class honours in English. Enlisting in 1916, she served as an orderly during World War I writing pieces for performance and using her colleagues and patients as cast members. Posted to France in 1917 Armstrong was awarded the Croix de Guerre for her bravery in rescuing wounded soldiers while under fire. She embarked for Australia on the Osterley on 21 May, 1919.
None of her war plays is known to have survived. 'The ephemeral nature of theatre put on for troops during the war means that few records remain of performances, let alone scripts (where they existed) for such productions' (Cullen, p.164). Armstrong briefly owned and operated the Amber Tea Rooms at Goulburn; then, in 1921, she took up a farming block - previously owned by her brother-in-law - under the Returned Soldiers' Settlement Scheme. Her sister Helen settled on an adjoining block. They both struggled financially despite their large land holdings and family support. After Helen's death in 1939, Millicent moved to a grazing property at Kirkdale, Yarra. By 1953 she was living at Goulburn.