In 1797, he was commissioned an ensign in the 52nd Regiment of Foot, in which his elder brother Robert (1780–1863) was also a captain, as recorded on the latter's tomb monument at St Nicholas Church in Carrickfergus.

Rowan saw active service during the Napoleonic Wars, in Sicily in 1806–1808, Sweden in 1808, and then in the Peninsular War, fighting at the Battles of Corunna, Ciudad Rodrigo, Badajoz, Buçaco, Fuentes d'Oñoro, the Côa and Salamanca, and serving as brigade major of the Light Brigade from 1809 and as assistant adjutant-general of the Light Division from 1811.

As regimental second-in-command, he commanded a wing of the 52nd Foot at the Battle of Waterloo, where he was wounded, and was made a Companion of the Bath (CB) for his services.

In June 1829, Rowan was selected by Home Secretary Sir Robert Peel as founding senior Commissioner of the Metropolitan Police after the first man offered the post, Lieutenant-Colonel James Shaw Kennedy, had turned it down.

They drew up regulations and pay scales, designed and ordered uniforms and equipment, and found, purchased and furnished station houses.

During the first two years of the force's existence, half of its original constables were dismissed, mainly for drunkenness, absenteeism, and frequenting pubs and fraternising with prostitutes while on duty.

The Commissioners felt obliged to dismiss the two officers for the good of the force, but refused to accept their guilt and wrote to the government in protest.

His principal hobbies were salmon fishing and shooting, often using as a base Floors Castle in Scotland, seat of his friend, James Innes-Ker, 6th Duke of Roxburghe.