Tennant was born in Russell Square, London, the second daughter of Charles Tennant and Gertrude Barbara Rich Collier (1819–1918). She studied painting under Edward Poynter at the Slade School of Fine Art, London and with Jean-Jacques Henner in Paris.
In 1890, she married the explorer of Africa, Henry Morton Stanley, and became known as Lady Stanley. She edited her husband's autobiography, reportedly removing any references to other women in Stanley's life.
Lord Henry Morton Stanley (1841–1904) (1880)