At the outbreak of the Indian mutiny he was given the command of the royal engineers of the force, under Lord Clyde. He took part in the operations at Cawnpore, in the siege and capture of Lucknow, and subsequent operations in Rohilkund and Oude. For his Indian services Harness was mentioned several times in despatches and was thanked by the governor-general in council. He was made a C.B., and received the medal and clasps.