The report submitted by him in February, 1817, was accepted as 'able, impartial, and satisfactory'.
After the termination of the war he served with his regiment at Allahabad, and in the neighbouring district of Partabgarh, where he laid the foundation of the intimate knowledge of Oudh affairs displayed in his later writings.
In 1820 he was selected for civil employ, and was appointed Junior Assistant to the Agent of the Governor-General, administering the Sagar and Nerbudda territories.

Those territories, which had been annexed from the Marathas two years previously, are now included in the jurisdiction of the Chief Commissioner of the Central Provinces.
In such a recently-conquered country, where the sale of all widows by auction for the benefit of the Treasury, and other strange customs still prevailed, the abilities of an able and zealous young officer had ample scope.

Sleeman, after a brief apprenticeship, received, in 1822, the independent civil charge of the District of Narsinghpur, in the Nerbudda valley, and there, for more than two years, 'by far the most laborious of his life', his whole attention was engrossed in preventing and remedying the disorders of his District.
Sleeman, during the time that he was in charge of the Narsinghpur District, had no suspicion that it was a favourite resort of Thugs.

A few years later, in or about 1830, he was astounded to learn that a gang of Thugs resided in the village of Kandeli, not four hundred yards from his court-house, and that the extensive groves of Mandesar on the Sagar road, only one stage distant from his head-quarters, concealed one of the greatest _bhils_, or places of murder, in all India.

The arrest of Feringheea, one of the most influential Thug leaders, having given the key to the secret, his disclosures were followed up by Sleeman with consummate skill and untiring assiduity.
In the years 1831 and 1832 the reports submitted by him and other officers at last opened the eyes of the superior authorities and forced them to recognize the fact that the murderous organization extended over every part of India.