In February, 1839, he assumed charge of the office of Commissioner for the Suppression of Thuggee and Dacoity.  Up to that date the office of Commissioner for the Suppression of Dacoity had been separate from that of General Superintendent of the measures for the Suppression of Thuggee, and had been filled by another officer, Mr. Hugh Eraser, of the Civil Service.  During the next two years Sleeman passed much of his time in the North-Western Provinces, now the Agra Province in the United Provinces of Agra and Oudh, making Muradabad his head-quarters, and thoroughly investigating the secret criminal organizations of Upper India.

In 1841 he was offered the coveted and lucrative post of Resident at Lucknow, vacant by the resignation of Colonel Low; but that officer, immediately after his resignation, lost all his savings through the failure of his bankers, and Sleeman, moved by a generous impulse, wrote to Colonel Low, begging him to retain the appointment.

Sleeman was then deputed on special duty to Bundelkhand to investigate the grave disorders in that province.  While at Jhansi in December, 1842, he narrowly escaped assassination by a dismissed Afghan sepoy, who poured the contents of a blunderbuss into a native officer in attendance.[3]

During the troubles with Sindhia which culminated in the battle of Maharajpur, fought on the 29th December, 1843, Sleeman, who had become a Lieut.-Colonel, was Resident at Gwalior, and was actually in Sindhia’s camp when the battle unexpectedly began.  In 1848 the Residency at Lucknow again fell vacant, and Lord Dalhousie, by a letter dated 16th September, offered Sleeman the appointment in the following terms: 

The high reputation you have earned, your experience of civil administration, your knowledge of the people, and the qualifications you possess as a public man, have led me to submit your name to the Council of India as an officer to whom I could commit this important charge with entire confidence that its duties would be well performed.  I do myself, therefore, the honour of proposing to you to accept the office of Resident at Lucknow, with especial reference to the great changes which, in all probability, will take place.  Retaining your superintendency of Thuggee affairs, it will be manifestly necessary that you should be relieved from the duty of the trials of Thugs usually condemned at Lucknow.  In the hope that you will not withhold from the Government your services in the capacity I have named, and in the further hope of finding an opportunity of personally making your acquaintance, I have the honour to be,
   Dear Colonel Sleeman,
     Very faithfully yours,
       DALHOUSIE.[4]