[3] Armstrong attended Northwestern University for a year and graduated from the Woman's Medical College of Pennsylvania in 1885, with an internship in gynecology and obstetrics at the Philadelphia Lying-in Charity Hospital.

[5] Armstrong founded and ran an orphanage in Platte County, Nebraska, as a young doctor.

She became a medical missionary in India as a single woman in 1886, serving with her sister Willimina L. Armstrong,[6][7] and later with her husband, Methodist clergyman George Armstrong-Hopkins.

[11][12] Thoburn had claimed that Armstrong-Hopkins was spending lavishly on dresses, stockings, shoes, and hats for her Indian patients.

[21] Her younger sister Willimina Leonora Armstrong was known later in life as Zamin Ki Dost, a physician, writer, and lecturer on Eastern mysticism, based in Los Angeles.