Promoted to Lieutenant on 26 May 1825, he fought at the Capture of Bharatpur later that year. He was made a Captain on 22 April 1826, and transferred to the 32nd (Cornwall) Regiment of Foot. After 10 years with the 32nd he moved to the 6th Dragoon Guards, before being given a year-long leave of absence in 1830 to serve a prison sentence.
Smyth had fought one of the last duels in the United Kingdom, and both he and his second were imprisoned for manslaughter.
After a series of promotions and successes he was made a Knight Commander of the Order of the Bath on 13 March 1867, promoted to Lieutenant-General on 1 April 1870, and became Colonel of the 6th Dragoon Guards on 21 January 1868.