Ambler was commissioned into the 3rd Battalion, the Prince of Wales’s Own (West Yorkshire Regiment), on 10 December, 1904. This was a Special Reserve Battalion, and so Ambler would not need to pass the entrance exam for Sandhurst. He exchanged, as Second Lieutenant, in to the 14th Hussars on 15 January, 1907, and was promoted Lieutenant on 29 September, 1909.

The 14th Hussars were in India when war broke out in August, 1914, and did not go to Mesopotamia until November, 1915. Meanwhile Lieutenant Ambler was seconded to the Royal Flying Corps on 7 June, 1915.