[1][3] Stewart was commissioned as an ensign in the 9th Bengal Native Infantry on 12 October 1840 and was promoted to lieutenant on 3 January 1844 and to captain on 1 June 1854.
[5] Promoted to major-general on 24 December 1868,[8] he became commandant of the penal settlement of the Andaman Islands, and was present when one of the inmates assassinated Lord Mayo, British Viceroy of India, in 1872.
[9] Promoted to lieutenant-general on 1 October 1877,[10] Stewart commanded a column during the Second Anglo-Afghan War advancing through the Bolan Pass to Quetta, and then on to Kandahar in January 1879.
[9] On hearing of the Maiwand disaster, he despatched Sir Frederick Roberts with a division on his celebrated march from Kabul to Kandahar, while he led the rest of the army back to India through the Khyber Pass.
[9] He returned to London to become a member of the Council of the Secretary of State for India in 1893 and, in that role, again argued - this time successfully - for the creation of a single Indian Army.
Lady Stewart was invested as a Companion of the Imperial Order of the Crown of India (CI) by Queen Victoria at Windsor Castle on 6 March 1900.