In 1812, at the age of fourteen, she found herself orphaned and only with a sister, when her home town Badajoz was besieged for the fourth time during the Peninsular War. After the siege ended in a successful but very bloody storming by the British and Portuguese forces, the sisters sought protection from the plundering and pillaging soldiers by some British officers they found camping outside the city walls. One of them was Brigade-Major Harry Smith, of the elite 95th Rifles scout regiment, whom she married a few days later.
Instead of letting herself be sent home to her husband's family, she chose to accompany him with the army. She remained with him throughout the rest of the war, accompanying the baggage train, sleeping in the open on the field of battle, riding freely among the troops, and sharing all the privations of campaigning. Her beauty, courage, sound judgment and amiable character endeared her to the officers, including the Duke of Wellington, who spoke of her familiarly as Juanita; and she was idolized by the soldiers.
With the exception of his stint in the British-American War of 1812 she accompanied her husband in all his deployments, most notably in two postings in South Africa, where Sir Harry (he had been knighted in the meantime) served as Governor of Cape Colony and High Commissioner.
Juana Smith was given a pension of £500 by Parliament on 5 December 1848 in recognition of her husband's services to the country.
Lady Smith is sometimes said to have introduced the cantaloupe or muskmelon (Cucumis melo cantalupensis) to South Africa, where it is known as spanspek (or spanspec or sponspe(c)k), which in Afrikaans literally means 'Spanish bacon' (Spaanse spek). However, the Oxford English Dictionary shows that the term 'Spanish bacon' had been in use since at least the eighteenth century.
Although it focuses primarily on her husband, Juana Smith is a central character in the 1984 book
The Other Side of the Hill by Peter Luke, which recounts their meeting and first few years of their marriage during the latter part of the Peninsular War and the Waterloo campaign.