A son of Henry Priestley Jacob, he was baptized at Poona, India, on 13th January 1871. He served in the Indian Army Political Depot. In November 1896 he married Jenny Coke Mickleburgh, daughter of Francis Hunt Mickleburgh and Gertrude Alice Coke; she was born about 1874 in Toronto, Canada. They had a daughter:

Arthur was gazetted 2nd Lieutenant as a Gentleman Cadet on 4th March 1891, Lieutenant 21st April 1893, when he joined the Staff Corps from the Gloucestershire Regiment, Major 4th March 1909; he was made CIE 22nd June 1914, and CSI 3rd August 1920. He was for some time Political Agent at Zhob, Baluchistan, India.

I have looked at his service record (British Library: IOR/R/1/4/1143). At the time when he was a Lieutenant nothing remarkable was said of him, other than that he is described as lacking initiative, as also being a bad horseman. By the time he had become Major it was said "Jacob is supposed to be without ambition, but I think he has the best ambition of all which is to earn the respect of the men he works among. He is one of the very best men in the department and is qualified to hold any position in it...." (13.11.1917).

A letter dated 31.8.1919 states "He practically refused the offer of the Revenue Commissionership, preferring to end his service in what he thought would be the continued peace and freedom of Zhob....". "He is quiet mannered, but firm and determined, his knowledge of the Pathans of Beluchistan in unrivalled and he is immensly liked and respected by them".

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