At the age of 14 years, Rowan was commissioned as an ensign in the 52nd Light Infantry on 4 November 1803 and promoted to lieutenant on 15 June 1804. He was deployed to Sicily in 1806 and to Sweden in 1808 before being promoted to captain and being given command of a company in the 2nd Battalion of his regiment on 19 October 1808.
Peninsular war and Walcheren
During the Peninsular War he fought in Spain under General Robert Craufurd: although heavily engaged providing covering fire for Sir John Moore's famous retreat, he was not present at the Battle of Corunna in January 1809, having been detached to Vigo, from where he returned to England. He was present at the capture of Flushing in August 1809 during the disastrous Walcheren Campaign. After returning to Spain, he was present at the Battle of Sabugal in April 1811, the Battle of Vitoria in June 1813, the Battle of the Pyrenees in July 1813 and the Battle of the Bidassoa in October 1813 as well as the Battle of Nivelle in November 1813, the Battle of the Nive in December 1813, the Battle of Orthez in February 1814 and, having been promoted to brevet major on 3 March 1814, he also fought at the Battle of Toulouse in April 1814. Rowan's promotion to brevet major was directly in recognition of his personal bravery and the success of his command in the fighting in the marsh which actually decided the Battle of Orthez in Wellington's favour.
Waterloo
During the Hundred Days Rowan fought at the Battle of Waterloo in June 1815, taking part in an important charge led by Sir John Colborne against the Imperial Guard, during which he was wounded in action and 150 of his men were killed or wounded. After the War he served in the Army of Occupation of France and was put in charge of the 1st arrondissement of Paris.
Canada
Promoted to brevet lieutenant colonel on 21 January 1819, Rowan was posted with his regiment to New Brunswick in 1823 before being promoted to the substantive rank of major on 4 May 1826. He transferred to the 58th Regiment of Foot on 27 July 1826 and, having been promoted to the substantive rank of lieutenant colonel on 22 July 1830, he became Military and Civil Secretary to Sir John Colborne, Lieutenant Governor of Upper Canada, in 1832. He was promoted to colonel on 10 January 1837 and assisted Colborne in Colborne's new role as Acting Governor General of British North America during the rebellions by the Patriote movement in 1837. Rowan was appointed a Companion of the Order of the Bath on 19 July 1838 before returning to England in 1839. Promoted to major-general on 9 November 1846, Rowan returned to Canada as Commander-in-Chief, North America in Spring 1849. In this role he made an important conciliatory speech in response to the burning of the Parliament Buildings in Montreal by an angry mob in April 1849. Rowan was promoted to the local rank of lieutenant general on 22 June 1849.
Rowan married Martha Spong of Aylesford, Kent, daughter of John and Rosamond Spong on 21 January 1811 at West Malling in Kent; they had no children. From at least 1860 he resided with his wife, a Chelsea Pensioner man-servant and household staff at No. 9 Gay Street, Bath.