Thomas Kelly was born on 27 February 1840 in Kilrush, County Clare, Ireland. He was educated as a lay student at St. Patrick's College, Carlow and at Sandhurst. He was the fifth son of Matthew Kelly and Mary Kenny He assumed in 1874 the additional name of Kenny, under the will of his maternal uncle, Dr. Mathias Kenny, a survivor of the Peninsular War and the Battle of Waterloo. He was appointed Ensign without purchase in the 1st Battalion, 2nd (The Queen's Royal) Regiment of Foot on 2 February 1858 and was appointed to command the escort of General Sir James Jackson General Officer Commanding Cape of Good Hope. When this officer was succeeded by General Wynward he was appointed ADC. He resigned this post on the outbreak of war with China in 1860 and accompanied his regiment to the Far East