In due course he was appointed to the office of General Superintendent of the Operations against Thuggee, which had been held by his uncle.

He rose to the rank of Colonel, and after a long period of excellent service, lived to enjoy nearly thirty years of honourable retirement.

He died at his residence near Ross in 1899 at the age of eighty-one.
In 1831 Sir William's only son, Henry Arthur, was gazetted to the 16th (Queen's) Lancers, and having retired early from the army, with the rank of Captain, died in 1905.
His elder son William Henry died while serving with the Mounted Infantry during the South African War.

His younger son, James Lewis, a Captain in the Royal Sussex Regiment, who also saw active service during the war, and was mentioned in dispatches, has a distinguished African and Indian record, and recently received the honorary degree of M.A.from the Belfast University for good work done in establishing the first Officers' Training Corps in Ireland.

The family of Captain James Lewis Sleeman consists of two sons and a daughter, namely, John Cuthbert, Richard Brian, and Ursula Mary.
Captain Sleeman, as the head of his family, possesses the MSS.