He was the son of Townsend Mullins, the second son of The 1st Baron Ventry, and his second wife Christabella, daughter of Solomon Dayrolles. Mullins was commissioned a lieutenant in the 7th Regiment of Foot on 5 February 1807, and served with the regiment in the Peninsular War. He fought at Busaco and Albuera and was badly wounded at the latter. He purchased a captaincy on 13 August 1811. Mullins was slightly wounded while serving with the 7th at the Battle of New Orleans, where his uncle, Thomas Mullins, commanded a regiment. He exchanged out of the 7th to take half-pay on 11 December 1817 and did not return to the Army.