[1] He attended the Royal Engineer Establishment at Chatham with the rank of ensign from 7 June 1827[2] before being promoted to lieutenant on 28 September 1827 and being sent to India in November 1828.

[3] Napier served under Sir Hugh Gough during the First Anglo-Sikh War and commanded the Bengal Engineers at the Battle of Mudki in December 1845.

[3] He was wounded during the siege but managed to recover sufficiently to be present at the successful storming of Multan in January 1849 and at the surrender of the fortress of Chiniot shortly thereafter.

[3] Napier was appointed military secretary and adjutant general to Sir James Outram, whose forces took part in the actions leading to the first relief of Lucknow in September 1857.

[6] Napier then joined Sir Hugh Rose as second-in-command for the march on Gwalior and commanded the 2nd Brigade at the Battle of Morar in June 1858.

[1] After Sir Hugh Rose's departure, Napier assumed command of the Gwalior division and helped capture Paori in August 1858, routed Prince Ferozepore at Ranode in December 1858 and secured the surrender of Man Singh and Tatya Tope, ending the war, in January 1859.

[1] In January 1860, during the Second Opium War, Napier assumed command of the 2nd Division of the expeditionary force under Sir James Hope Grant.

[1] The expedition involved crossing 400 miles (644 km)[14] of mountainous terrain lacking roads or bridges occupied by local people with a known history of hostility towards outsiders.

[15] On the one hand, Napier made it clear to the Ethiopians that the sole intent of the British force was to rescue the imprisoned Europeans—not conquest; on the other, Napier met with local potentates such as Ras Kassa (the future Emperor Yohannes IV) and arranged to purchase needed supplies with the 4.35 million Maria Theresa thalers (the preferred currency of the area) the British had purchased from the mint in Vienna.

The British captured Magdala, and Emperor Tewodros killed himself, leaving a grandiose statement that he preferred to "fall into the hands of God, rather than man.

In November 1879 he represented Queen Victoria at Madrid as ambassador extraordinary upon the occasion of Alfonso XII of Spain's second marriage and in December 1879 he became a member of the Royal Commission on the organization of the army.