Thither he proceeded as soon as possible, and was promoted successively to the rank of Ensign, 23rd September, 1810; Lieutenant, 16th December, 1814; Brevet-Captain, 24th April, 1824; Captain, 23rd September, 1826; Major, 1st February, 1837; Lieutenant-Colonel, 26th May, 1843; Colonel, 24th November, 1853; and obtained the rank of Major-General 28th November, 1854.
Early in his career he served in the Nepaulese war.

The value of his talents soon became known, and in 1816, when it was considered necessary to investigate a claim to property as prize-money arising out of that war, Lieutenant Sleeman was selected to inquire into it.
The report was accordingly made by him in February 1817, which was designated by the Government as "able, impartial, and satisfactory." In 1820 he was appointed junior Assistant to the Agent of the Governor-General at Saugur, and remained in the Civil Department in the Saugur and Nerbudda territories, with the exception of absence on sick certificate, for nearly a quarter of a century.

Here he manifested that, if he had been efficient in an inferior position, he was also an able administrator in a superior post.

He distinguished himself so much by his activity in the suppression of the horrible practice of Thuggism, then so prevalent, that, in 1835, he was employed exclusively in the Thuggee Department; his appointment in the Saugur and Nerbudda districts being kept open, and his promotion going on.

The very valuable Papers upon Thuggism submitted to the Governor-General were chiefly drawn up by Sir William Sleeman, and the department specially commissioned for this important purpose was not only organised but worked by him.