After initial service with the Bombay Staff Corps in the Public Works Department, and a brief spell with the Aden Field Force in 1865–6, he was appointed in 1867 as chief engineer of the state of Jaipur in Rajasthan, India.

[6] During the summer of 1902, he accompanied the Maharaja of Jaipur as a political officer during a visit to the United Kingdom to attend the Coronation of King Edward VII and Queen Alexandra.

[10][11] He had no sooner retired to England in 1911 then he was recruited by the secretary of state for India to assist Edwin Lutyens and Herbert Baker in the design of New Delhi.

[1] Jacob's department was responsible for the construction of everything in the state of Jaipur ranging from walls, outhouses, guard houses, roads, canals to major public buildings.

As a result, he became – with F. S. Growse, Robert Fellowes Chisholm, Charles Mant, Henry Irwin, William Emerson, George Wittet and Frederick Stevens – a pioneer of the Indo-Saracenic style of architecture.