He came to acquire the Jami" al-tawarikh in its original manuscript. In his studies of the Roma people, he made an identification with a legend of Bahram Gur and the Luri to support a Romani presence in Sasanid Persia, now considered to be an unjustified and uncritical deduction that has persisted. In 1796 he became a cadet with the East India Company, and became a lieutenant in the Bengal Presidency in 1798, captain in 1806, and major in 1817.
Lieutenant-colonel in 1823 and colonel in 1829.
In 1803 he lost a leg at the Battle of Delhi, serving under Lord Lake. He lived much of his life in Calcutta, and ultimately reached the rank of Major General.
In 1819-1820 Harriott was collecting Romani vocabulary in Hampshire, England. Some of his results were read to the Asiatic Society of Bengal in 1822.
In 1830 a membership list for the Royal Asiatic Society gives his address as Mortlake.
Harriott is known as one the owners of the key manuscript of the Jami" al-tawarikh, and important medieval manuscript dealing with Mongol history. How he came into its possession is not clearly known. According to Sotheby"s, he found it in Danapur in 1813.
William Hook Morley noticed it in the Royal Asiatic Society"s manuscript collection in 1838.
From 1948 it was at the British Museum, on loan, and then was sold, to an unknown purchaser.