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.  In consequence of ill-health, however, at the end of 1836, he was compelled to resign this appointment; but on his return to duty in February 1839, he was nominated to the combined offices of Commissioner for the Suppression of Thuggee and Dacoity.

In 1842 he was employed on a special mission in Bundelcund, to inquire into the causes of the recent disturbances there, and he remained in that district, with additional duties, as Resident at Gwalior, from 1844 until 1849, when he was removed to the highly important office of Resident at the Court of Lucknow.  Colonel Sleeman held his office at Gwalior in very critical times, which resulted in hostilities and the battle of Maharajpore.  But for a noble and unselfish act he would have received this promotion at an earlier period.  The circumstance was this:  Colonel Low, the Resident at that time, hearing that his father was dangerously ill, tendered his resignation to Lord Auckland, who immediately offered the appointment to Colonel Sleeman.  No sooner had this occurred, however, than Colonel Low wrote to his Lordship that, since he had resigned, the house of Gaunter and Co., of Calcutta, in which his brother was a partner, had failed, and, in consequence, every farthing he had saved had been swept away.  Under this painful contingency be begged to place himself in his Lordship’s hands.  This letter was sent by Lord Auckland to Colonel Sleeman, who immediately wrote to Colonel Low, begging that he would retain his situation at Lucknow.  This generous conduct of Colonel Sleeman was duly appreciated; and Lord Auckland, on leaving India, recommended him to the particular notice of his successor.  Lord Ellenborough, who immediately appointed Colonel Sleeman to Jhansi with an additional 1000_l_. a-year to his income.

Colonel Sleeman held the appointment of Resident at Lucknow from the year 1849 until 1856.  During this period his letters and diary show his unwearied efforts to arrive at the best information on all points with regard to Oude.  These will enable the reader to form a just, opinion on the highly-important subject of the annexation of this kingdom to British India.  The statements of Colonel Sleeman bear inward evidence of his great administrative talents, his high and honourable character, and of his unceasing endeavours to promote the best interests of the King of Oude, so that his kingdom might have been preserved to him.  Colonel Sleeman’s views were directly opposed to annexation, as his letters clearly show.