In 1890, it was reconstituted with Trans-frontier Pathans to become the only all-Pathan regiment in the Indian Army; acquiring in the process, the nickname of "Forty Thieves".

Subsequent to the reforms brought about in the Indian Army by Lord Kitchener in 1903, the regiment's designation was changed to 40th Pathans.

In December, the regiment left for East Africa, where they served till February 1918, and again distinguished themselves in the long and bitter campaign.

The 40th Pathans suffered a total of 800 casualties (killed or wounded) during the war, while 1066 officers and men were invalided out of service due to sickness.

[3] During the Second World War, the battalion fought in the Malayan Campaign and was taken prisoner by the Japanese on Singapore Island following the British surrender on 15 February 1942.